How to calculate molecular formula

[Gokul43201]

Yes it's the valency. If element A has valency x and element B has valency y. Then the compond formed by them will be $$A_yB_x$$
ie. interchange the valencies. Now if x,y have some common factor, then you reduce this to the simplest ratio.

eg : Zn = 2; O = 2 => Compound = Zn2O2 = ZnO

Some elements exhibit more than one valency, though one of these is more common than the other. If not specified, you should assume the more common valency. But most sensible people specify the valency in such cases. eg: Fe(III), Cr(VI), Zn(IV)

Valencies of common radicals : NO3 = 1, SO4 = 2, PO4 = 3, CO3 = 2, OH = 1, SO3 = 2, NH4 = 1

PS : Check your double-decomposition reaction, and correct it.

 

[BH20]

Yes it's the valency. If element A has valency x and element B has valency y. Then the compond formed by them will be $$A_yB_x$$
ie. interchange the valencies. Now if x,y have some common factor, then you reduce this to the simplest ratio.

eg : Zn = 2; O = 2 => Compound = Zn2O2 = ZnO

Some elements exhibit more than one valency, though one of these is more common than the other. If not specified, you should assume the more common valency. But most sensible people specify the valency in such cases. eg: Fe(III), Cr(VI), Zn(IV)

Valencies of common radicals : NO3 = 1, SO4 = 2, PO4 = 3, CO3 = 2, OH = 1, SO3 = 2, NH4 = 1

PS : Check your double-decomposition reaction, and correct it.

You do the crossover though..right?

For example in that first example:

Zinc is +2, SO4 is -2 Ba is +2 NO3 is -1

So, let's look at ZnSO4. Since Zinc has a valency of +2, it will go across and should really be ZnSO4(2) and then, since SO4 is -2, the total should be Zn2SO42. As you said though, we want to reduce to the simplest ratio, so since its both 2's, we can just leave it like that, it would reduce to 1..right?

And then if we look at BaNO3...Barium is +2, so we crossover and it should make it NO3(2), NO3 has a valency of -1, so that means we don't put anything infront of Ba..so its BaNO3(2).

ZnSO4 + BaNO3(2) produces ZnNO3(2) + BaSO4

Is this right..and it already looks balanced..the question states to balance it too..so maybe its wrong?

One small issue for the elements with more valency's...is that I'm not really sure which one is the common.

 

[Gokul43201]

So far so good ! And it is balanced, so there's nothing more to do there.

"I'm not sure which valency is more common"

Nor am I...that's why it's usually specified. But for Zn, II is more common than IV, and for Cr, II and III are more common than VI. Fe II and III are both fairly common. Cu, II more than I. It's only with the transition metals (d-block) that you see these multiple valencies.

 

[BH20]

ok..thanks

The second has:

Gold(III) Sulphate + Barium Chloride produces

Does the (III) mean we use the +3 valency, and if so, do we cross that over too?

So it would be:

Au2SO4(3) + BaCl2 produces Au2Cl3 + BaSO4

Does it look right, and if so, how would we balance it?

 

[Gokul43201]

ok..thanks

The second has:

Gold(III) Sulphate + Barium Chloride produces

Does the (III) mean we use the +3 valency, and if so, do we cross that over too?

Yes, always !

So it would be:

Au2SO4(3) + BaCl2 produces Au2Cl3 + BaSO4

Does it look right, and if so, how would we balance it?

Mistake on the right side...find it. Then we can get to balancing.

 

[Gokul43201]

For balancing an equation, you need to make sure there are an equal number of atoms of each element on the LHS and RHS. You cannot change the molecular formula to get this right. What you can do is change the coefficients (number of moles/molecules) before each compound.

 

[BH20]

dont see the mistake...Au is +3 (in this case anyways) Cl +2..so Au2Cl3
and then Ba is +2 and SO4 is -2, so it cancels out.

 

[Gokul43201]

Is Cl really 2 ? Then how did you get BaCl2 ?

 

[Gokul43201]

Cl valency is -1, not -2.

 

[BH20]

Cl valency is -1, not -2.

yep, my bad..dont know what made me think nit was -2.

Au2SO4(3) + BaCl2 produces AuCl3 + BaSO4

LS has to equal RS

LS= Au=2, SO4=3, Ba=1, Cl=2
RS= Au=1, SO4=1 Ba =1, Cl=3

Balanced:

Au_2(SO4)_3 + 3BaCl2 produces 2AuCl_3 + 3BaSO4

LS= Au=2, SO4=3, Ba=3, Cl=6
RS= Au=2, SO4=3, Ba=3, Cl=6

looks good to me.

 

[Gokul43201]

And to me.

 

[BH20]

I have some other ones to do which are different, more complex:



1. Zinc + mercurous nitrate produces - single displacement reaction.

Zn + HgNO3 produces Hg + Zn(NO3)_2

Balance:

Zn + 2HgNO3 produces 2Hg + Zn(NO3)_2

2. tin + Antimony(V) chloride produces - single displacement reaction.

Sn + SbCl4 produces Sb + SnCl4

Balance:

Looks fine. Altought Tin does have a valency of +2 and +4, but I think this one is right.

3. Calcium + water produces - synthesis reaction

Ca + H20 produces

not sure how this one comes together...pretty sure its not CaH20.

Found this on the net, but not sure why its like that:

Ca(s) + 2H2O(l) ——> Ca(OH)2(aq) + H2(g

Have a few more I will bring up.

 

[BH20]

4. Ferrous Bromide + Phosphoric Acid - not sure what type of reacion

FeBr + H3O4P produces

no idea how to do this one...

5. the complete combustion of ethene, C2H4 produces -

not sure either

6. calcium carbonate + hydrobromic acid produces - guess double displacement?

Ca2(Co3)_3 + HBr produces not sure

 

[Gokul43201]

I have some other ones to do which are different, more complex:



1. Zinc + mercurous nitrate produces - single displacement reaction.

Zn + HgNO3 produces Hg + Zn(NO3)_2

Balance:

Zn + 2HgNO3 produces 2Hg + Zn(NO3)_2

2. tin + Antimony(V) chloride produces - single displacement reaction.

Sn + SbCl4 produces Sb + SnCl4

Balance:

Looks fine. Altought Tin does have a valency of +2 and +4, but I think this one is right.

3. Calcium + water produces - synthesis reaction

Ca + H20 produces

not sure how this one comes together...pretty sure its not CaH20.

Found this on the net, but not sure why its like that:

Ca(s) + 2H2O(l) ——> Ca(OH)2(aq) + H2(g

Have a few more I will bring up.

Always treat H2O as though it were written HOH, with H (valency +1) being the cation, and OH(valency -1) being the anion.

So, Ca + HOH ---> Ca(OH)2 + H but then H exists only as H_2
So, Ca + H2O ---> Ca(OH)2 + H2, and now balance this to get the above equation.

PS : When a metal exhibits more than two valencies (eg : Fe II and III), the lower valency is indicated by an -ous ending and the higher valency by an -ic ending. So Fe(II) is called ferrous and Fe(III) is calles ferric.

That's how you should know that the mercurous in (Q1) above is Hg(I) and not Hg(II)

 

[Gokul43201]

Acids are made up of a H (cation) and any anion that is usually among Cl, Br, SO4, PO4, NO3, etc. The number of H atoms depend on the valency of the anion.

PO4 is -3. So phosphoric acid is H3PO4.
SO4 is -2. So sulphuric acid is H2SO4.
Cl, Br, NO3 are all -1. So you have HCl (hydrochloric), HBr (hydrobromic) and HNO3 (nitric acid).

Likewise, a base is usually made up of any common cation (Ca, Na, Mg, K, Cu, Zn, etc.) and OH (-1) as the anion. They are named as hydroxides. eg : Ca(OH)2 is calcium hydroxide, NaOH is sodium hydroxide.

When a hydrocarbon combusts, it is reacting with oxygen (O2) to give CO2 and H2O. So hydrocarbon + O2 ---> H2O + CO2. All you have to do is balance this. (Hint: First balance C and H, then do O)

 

[BH20]

Acids are made up of a H (cation) and any anion that is usually among Cl, Br, SO4, PO4, NO3, etc. The number of H atoms depend on the valency of the anion.

PO4 is -3. So phosphoric acid is H3PO4.
SO4 is -2. So sulphuric acid is H2SO4.
Cl, Br, NO3 are all -1. So you have HCl (hydrochloric), HBr (hydrobromic) and HNO3 (nitric acid).

Alright, at least I get how it comes together now, (like why its H3PO4, for some reasons I thought it was different because it was an acid) but I'm still not sure if I understand what it produces.

The first one for example:

FeBr + H3PO4 produces FePO4 + BrH3 ?

if you can show me that one..I usually get a hang of it when I see it.

 

[Gokul43201]

Remember...H is (almost) always a cation (+ve valency). The convention is put put the cation before the anion (-ve valency). So it would be HBr and definitely not BrH3. Where did the 3 come from ? You're forgetting to assign the valencies and switch their positions.

Ferrous bromide should be FeBr2, since Fe is +2 (not +3, that would be ferric)
Phosporic acid is H3PO4.

They react via double-decomposition...so the products are Fe3(PO4)2 and HBr.

Now balance this.

 

[BH20]

Remember...H is (almost) always a cation (+ve valency). The convention is put put the cation before the anion (-ve valency). So it would be HBr and definitely not BrH3. Where did the 3 come from ? You're forgetting to assign the valencies and switch their positions.

Ferrous bromide should be FeBr2, since Fe is +2 (not +3, that would be ferric)
Phosporic acid is H3PO4.

They react via double-decomposition...so the products are Fe3(PO4)2 and HBr.

Now balance this.

3FeBr2 + 2H3PO4 produce Fe3(PO4)2 + 6HBr

?

 

[BH20]

Not sure about question 5:

5. the complete combustion of ethene, C2H4 produces -

I think I got the last one though:

6. calcium carbonate + hydrobromic acid produces - guess double displacement?

Written and Balanced: Ca2(Co3)_3 + 2HBr produces 2CaBr + H2CO3

 

[Gokul43201]

3FeBr2 + 2H3PO4 produce Fe3(PO4)2 + 6HBr

?

Correct...!

 

[Gokul43201]

Not sure about question 5:

5. the complete combustion of ethene, C2H4 produces -

I think I got the last one though:

6. calcium carbonate + hydrobromic acid produces - guess double displacement?

Written and Balanced: Ca2(Co3)_3 + 2HBr produces 2CaBr + H2CO3

6. How is it Ca2(Co3)_3 ? What is the valency of the CO3 radical ?

5. See bottom of post#50. C2H4 is a hydrocarbon - a compound having carbon and hydrogen.

 

[BH20]

oh, made a mistake...Ca is +2, and CO3 is -2. Here it is.

6. 2CaCO3 + 2HBr produces 2CaBr + H2CO3

and 5...

C2H4 + 202 produces 2H2O + CO2 ??

 

[Gokul43201]

oh, made a mistake...Ca is +2, and CO3 is -2. Here it is.

6. 2CaCO3 + 2HBr produces 2CaBr + H2CO3

and 5...

C2H4 + 202 produces 2H2O + CO2 ??

Umm, CaBr ? Check that.

And for 5. recheck the Carbons.. you have 2 on the LHS and only 1 on the RHS. Use the Hint from #50.

 

[BH20]

Umm, CaBr ? Check that.

And for 5. recheck the Carbons.. you have 2 on the LHS and only 1 on the RHS. Use the Hint from #50.

5. C2H4 + 302 produces 2H2O + 2CO2

6. CaCO3 + 2HBr produces CaBr2 + H2CO3

 

[BH20]

I've done a lot of other questions, but I want to check with you if I did them properly..so some short questions for you, to see if I did them properly.

1. Asked me to find the number of atoms...

a) gave me just 52.5g of calcium metal..I put the g over the molarmass and then * it by 6.02* 1023molecules.
b) and c) parts gave me the number of moles, so I found the mass, and then again * by 6.02*1023

2. There was a question that asked to find the volume of a gas at -30degreescelsius and 0.75atm if it occupies a volume of 255L at 40degreescelcius and 1.25atm... I used the General Gas Equation. My question is, is it V1 or V2 that wer are finding..V1 right?

3. Also it asked me to compare a mixture of hydrogen and chlorine in a cylinder (without doing calculations), when it comes to:
a) relative mass..I said Cl is heavier (has bigger atomic mass)
b) average kinetic engery..I said Cl molecules have a higher kinetic enegery because its heavier.
c) the average speed...I said it is dependent on the temperature..but I'without doing calculations I don't know where you can find that info.
d)the temperature..not sure where to find that, but I said, Hydrogen is hotter because its a non-metal.

 

[Gokul43201]

1a) Okay (b,c) If you have the # of moles, you don't need to find the mass. Just multiply the # of moles by N = 6.02*10^23 atoms/mole, to get the # of atoms.

2) Depends on what you mean by V1 and V2. What did you use for P1, T1, P2, T2 ?

3a) Correct.
(d) "Hydrogen is hotter because it is a non-metal." Heavens, NO. What about chlorine ? It is also a gas. They are put together into a cylinder, so they mix with each other and get to be the same temperature. When 2 things are in contact for enough time they reach a common temperature.
(b) Temperature is just a measure of the Average Kinetic Energy of the molecules. Since their temperatures are equal, so must be their KEs. KE = (1/2)mv^2
(c) Since their KEs are equal and the Cl2 molecule has a greater mass, it must have a lower speed.

PS : Read Ch 6 (Kinetic Th. of Gases) in the link I gave you before.

 

[BH20]

1a) Okay (b,c) If you have the # of moles, you don't need to find the mass. Just multiply the # of moles by N = 6.02*10^23 atoms/mole, to get the # of atoms.

2) Depends on what you mean by V1 and V2. What did you use for P1, T1, P2, T2 ?

3a) Correct.
(d) "Hydrogen is hotter because it is a non-metal." Heavens, NO. What about chlorine ? It is also a gas. They are put together into a cylinder, so they mix with each other and get to be the same temperature. When 2 things are in contact for enough time they reach a common temperature.
(b) Temperature is just a measure of the Average Kinetic Energy of the molecules. Since their temperatures are equal, so must be their KEs. KE = (1/2)mv^2
(c) Since their KEs are equal and the Cl2 molecule has a greater mass, it must have a lower speed.

PS : Read Ch 6 (Kinetic Th. of Gases) in the link I gave you before.

1 b) gave me 1.5mol of carbon, and wanted me to find the number of atoms.
c) gave me 1.5mol of Carbon Dioxide to find the number of atoms.

the mm should be in this though..no? because we just have the moles of carbon..or am I getting mixed up?

2. Original Question: Find the volume of a gas at -30degreescelsius and 0.75atm if it occupies a volume of 255L at 40degreescelcius and 1.25atm... I used the General Gas Equation.

V1=?
T1=-30dc=243K
P1=0.75atm
V2=255L
T2=40.0dc=313K
P2=1.25atm

P1V1T2=P2V2T1

??

3d) oops..yeah, you are right, that made no sense.

 

[Gokul43201]

1) There is no place for the molar mass in (b,c). A mole is simply a quantity equal to a fixed number (the Avogadro Number, N(av) = 6.02*10^23) of atoms/molecules. So if you have 1 mole of anything, there are N(av) number of atoms/molecules in it.

In (1a) you used the mass and molar mass to find the # of moles. In (b,c) you are already given the # of moles, so why would you want to use the molar mass ?

2) Correct.

 

[BH20]

In (b,c) you are already given the # of moles, so why would you want to use the molar mass ?

I guess its a trick question then?

because b) had 1.5 mol and c) had 1.5mol..so would we not get the same answer for b and c?

but b) had 1.5 mol of carbon and c) had 1.5 mol of carbon dioxide

so that's where I got mixed up I guess.

 

[Gokul43201]

Yes, it's the same answer. The idea of the question is to reinforce the concept of a mole.

 

[BH20]

doing more questions here, stuck on one...

What experimental evidence requries that the emission of energy by an atom be quantized?

 

[BH20]

and on another question, I answered that the atomic number has more to do with the chemical behaviour of an element than atomic mass, because the atomic number is also the number of electrons an element has, and electrons dictate the type of bond and behaviour of the element.

 

[Gokul43201]

Yes, that's right.

and look up the Photoelectric Effect.

 

[BH20]

What could you tell me about Neutralization reactions, and ionic equations?

I'll jist give one question from each:

Neutralization:

Write equations for the neutralizations reactions that result in the formation of the following salts:

a) lithium carbonate

Ionic equations:

Identify those for which a reaction is likley to occur. For those that do occur, write a net ionic equation.

a) Chromium dipped into silver nitrate.

 

[Gokul43201]

Neutralization : acid + base --> salt + water

eg : H2SO4 + 2NaOH --> Na2SO4 + 2H2O

Recall my previous description of acids and bases. The reaction is basically a double-displacement reaction. Notice how the salt directly tells you what acid and base were used.

Ionic equation likely to occur : Look up the (Pauling) electronegativity series. There are 2 basic (equivalent rules). They are :

1. A more electropositive (or less electronegative) cation will displace a less electropositive (or more electronegative) cation. eg : Na(En=0.9) will displace Cu(En=1.9). The reverse will not happen.

2. A more electronegative anion will displace a less electronegative anion. eg : F(En=4) will displace Br(En=2.8).

In short, electropositive cations and electronegative anions are better at forming compounds.