How to Calculate Mass in Stoichiometry Problems?

[Samantha]

1. Aluminum chloride (15.0g) + sodium hydroxide (15g) = double displacement reaction.Predict mass of aluminum hydroxide produced. What mass of the excess reagent remains unreacted?

2. What mass of copper(II) sulfate pentahydrate is needed to prepare 150.0 mL of a 0.125mol/L solution?

How do I find them?

 

[MusicMonkey]

2. convert 150 ml to L then multiply Volume * Concentration. Therefore = (0.150 L)(0.125 mol/L). This will get you the number of moles of copper (II) sulfate pentahydrate. Then multiply the Molecular Weight of copper (II) sulfate pentahydrate by the number of moles and you will get the number of grams. Hope that helps.

 

[Samantha]

does the Molecular Weight of copper (II) sulfate pentahydrate = 164.21g/mol?

 

[MusicMonkey]

Not sure, if you calculated it using the molecular weights then it's probably right.

 

[Samantha]

Not sure, if you calculated it using the molecular weights then it's probably right.

copper(II) sulfate pentahydrate

This is what I did:

(molar mass of Cu * 2 ) + ( molar mass of S) + (molar mass of H * 5

 

[MusicMonkey]

Cu+S+O*4+5*(H*2+O)

The molecular formula is CuSO4(H2O)5

 

[MusicMonkey]

Therefore your molecular weight is incorrect. Try again.

 

[Samantha]

Therefore your molecular weight is incorrect. Try again.

I re-calculated it and it came out to 249.71g/mol??

 

[dextercioby]

I re-calculated it and it came out to 249.71g/mol??

It should come out 250.

What about the first problem??Can u do it...?

Daniel.

 

[MusicMonkey]

63.546+32.066+15.9994*4+5*(1.00794*2+15.9994)
I believe it should come out to be 249.686

 

[Samantha]

1. Is this right:

Mol of AlCl3 = 0.112mol
Mol of NaOH = 0.125mol

mass = (0.112mol)(78.03g/mol)
= 8.74g

b)do I multiply 0.125 mol by the mols of 3NaCl. Then subtract 8.74 from the anwser?

 

[MusicMonkey]

Depending on the number of the significant figures used I believe that your answer should be accurate enough.

 

[MusicMonkey]

The final answer for question 2 should be 4.68 g.

 

[dextercioby]

63.546+32.066+15.9994*4+5*(1.00794*2+15.9994)
I believe it should come out to be 249.686

No,no,no,no,no.No such precision required whatsoever.YOUR FINAL ANSWER IS APPROXIMABLE TO 250...

Daniel.

 

[dextercioby]

1. Is this right:

Mol of AlCl3 = 0.112mol
Mol of NaOH = 0.125mol

mass = (0.112mol)(78.03g/mol)
= 8.74g

b)do I multiply 0.125 mol by the mols of 3NaCl. Then subtract 8.74 from the anwser?

The remaining mass is of NaOH indeed & is roughly 1.5g

Daniel.

 

[Samantha]

The remaining mass is of NaOH indeed & is roughly 1.5g

Daniel.

How did you get that?

 

[dextercioby]

1 mole AlCl_{3}----------------->133.5g
1 mole NaOH-------------------------> 40g

The chemical reaction:
$$3NaOH+AlCl_{3}\rightarrow Al(OH)_{3}+3NaCl$$

So
3 moles NaOH/120g react with 1mole AlCl_{3}/133.5 g.
x g NaOH react with 15g of AlCl_{3}

Find "x" & subtract it from 15.

Daniel.

 

[Samantha]

1 mole AlCl_{3}----------------->133.5g
1 mole NaOH-------------------------> 40g

The chemical reaction:
$$3NaOH+AlCl_{3}\rightarrow Al(OH)_{3}+3NaCl$$

So
3 moles NaOH/120g react with 1mole AlCl_{3}/133.5 g.
x g NaOH react with 15g of AlCl_{3}

Find "x" & subtract it from 15.

Daniel.

 

[Gokul43201]

Samantha, before solving problems on stoichiometry, you need to learn how to write and balance equations. I suggest you first work on this, and get your basics clear. It may take a while but it's very important, and essential to your being able to solve any kind of chemistry problem.

 

[Samantha]

Samantha, before solving problems on stoichiometry, you need to learn how to write and balance equations. I suggest you first work on this, and get your basics clear. It may take a while but it's very important, and essential to your being able to solve any kind of chemistry problem.

I got the same equation but I don't get the calculations..I should work on that.

 

[Samantha]

I tried it and I got 1.5g

Thank you