# Chemistry review questions-calculations

1. Jun 17, 2006

### Aya

Hi, I have some reviw questions for the upcomming chemistry exam, the problem is I don't have the answers so can some one read throught my work and check if it is wright? Thanks.
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1. An anaysis of a valitile liquid shoed that it wsa composed of 14.4%C, 2.37% H and 83.49% Cl. If 4.25 g of the liquid was vaporized and occupied 628 mL at 65 degrees celcuios and 112 kpa, what would be the molecular formula of the compund?
carbon
14.4% x 100g
= 14.4 g

m=14.4 g
n=1.199 mol
Mm=12.01 g/mol

hydrogen
2.37% x 100g
=2.37g

m=2.37 g
n=2.347mol
Mm=1.01 g/mol

Cholrine
83.49% x 100g
=83.49g

m=83.49 g
n=2.355mol
Mm=35.45 g/mol

so...

1.199 / 1.199 =1
2.37 / 1.1.99=2
2.355/ 1.199=2

Impirical Formula
C1 H2 Cl2

# mol of Impirical formula
PV=nrt
n=PV/rt
n=(112Kpa)(0.628L)/ 8.314kpaL/molL (388k)
n=0.025 mol

Molar mass of Impreical formula
Mm=m/n
Mm=4.25g/0.025 mol
Mm=170 g/mol

Molar mass of ( what is this the molar mass of???)
C1 H2 Cl2
84.93

Find a Factor

170g/mol/ 84.93 g/mol
=2

Molecular Formula
2(C1 H2 Cl2)

C2 H4 Cl4

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2. A gas has a volume of 40.0 ml at 40 degrees celcuios and 95 Kpa. What will be its volume at STP?

gas
40.0 mL = 0.04L
40 degrees celcioue=313 K
95 kPa=P

n=rt/pv
n= 8.134(313)/95(0.04)
n=27.3890 mol

v=nrt
v=27.3809(8.134)(273)/101.3
v=613.4948
***
3. How many grams of sodium and required to product 2.24 L of Hydrogen gas, measured at 25 degrees celcuoiu adn 110 Kpa, according to the following reaction?

2Na + 2 H2O ---> 2 NaOH + H2O

n=rt/pv
n= 8.314(298)/110(2.24)
n=10.055

m=nMm
m=20.11 (22.99)
m=462.3289

B)calculate the volume of H2(g) produced when 54 g of Na(s) reacts with an excess of water at STP.

moles of Na
n=m/Mm
n=54/22.99
n=2.348

moles H
3.348/2
1.174

Volume
v=nrt/p
1.174(8.314)(273)/101.3
v=2.791

***
4. An aquois solution has a volume of 2.0L and contains 36.0 g of glucose. If the molar mass of glucose is 180 g , what is the molarity of the solution?

molar mass of glucose
n=m/Mm
n=36.0/180
n=0.2

...what next?
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5. How would you prepare 100ml of 0.40 mol/L MgSO4 form a stock solution of 2.0 mol/L MgSO4?
n=0.100L/0.40 mol/L
0.25

v=n/c
v=0.25/2.0
v=0.125L
***
6. A solution contains 5.85 g of sodium chlride dissolved in 5.00 x 10^3 ml of water. What is the concentration of the sodium cholirde solution?
n=M/Mm
n=0.0568 mol

C=n/v
0.0568/5
=0.01136 mol/L
***
7.what mass of potassium hydroxide is required to prepare 6.00 x 10^2 ml of a solution with a concentration of 0.225 mol/L
m=cv
0.225(0.5)
0.135mol

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8. What volume of 0.500mol/L sodium hyroxide solution can be prepared form 10.0 ml of a 6.00 mol/L solution?
C1V1=C2V2
6(0.01)=0.500(v2)
0.12=v2

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9. What is the mass of hydrochoric acid that is present in 500 mL of a solution containg 3.50 mol/L of HCl(aq)?
n=cv
3.50(0.5)
=1.75 mol

m=nMm
1.75(36.46)
63.805g

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10. Passing a park throught a mixture of hydrogen gas and oxygen gas produced water

a) calculate the mas of hydrogen needed to completly convert 4.00 g of oxygen into water

H2 + O = H2O

moles of O
n=m/Mm
4.00/18.025
0.22

moles of H
0.22 mol

mass of H
m=nMm
=0.22(1.01)
=0.224

b) calcute the number of moles of oxygen required to react with 12.5 moles of hydrogen gas
I dont know how to do this one
c)calculate the number of moles of water produced when 4.00 g of oxygen are used

I dont know how to do this one

***

Thanks for reading pleas post any corrections and solutions to the problems I don't know how to do. this is practise for an exam so pleas help!!!!

2. Jun 18, 2006

### Hootenanny

Staff Emeritus
HINT:

$$\text{M} = \frac{\text{moles}}{\text{volume}}$$

You have almost already written the answer in your post, look again at the equation you written, i would just like to correct it however;

$$H_{2}_{(g)} + \frac{1}{2}O_{2}_{(g)} \rightarrow H_{2}O_{(l)}$$

Now, all the answers you need are in the above equation.

Last edited: Jun 18, 2006
3. Jun 18, 2006

### Aya

4.
M=Mol/vol
M= 0.2mol/2.0L
M=0.1mol/L
***
The teacher said we should only balance with whole nubbers so would it be

2H2 + O2 = 2H2O....?

moles of H
0.22 mol
sooo... moles of O is also 0.11mol?

c)calculate the number of moles of water produced when 4.00 g of oxygen are used

2H2 + O2 = 2H2O
2 : 1 = 2

oxygen
n=M/mm
n=4.00g/16.0
n=0.25mol

Water
0.25mol x 2
=0.5 mol

Are these ones right, and is everything elce right?

4. Jun 18, 2006

### Hootenanny

Staff Emeritus
Spot on.
If your teacher restricts you to using whole numbers, then yes that is correct, however using half a diatomic molecules is acceptable.
In you original post you said, 12.5 mols of hydrogen.
Yes, that's spot on! I'm afraid I haven't checked the other as I haven't really got time, my apologies; but if you check back later I'm sure someone will have obliged.

5. Jun 18, 2006

### Aya

^ Oh, ok thanks for all your help!