General Chemistry, Chapters 4-6

Practice ProblemsDr. Badger

1. A solution contains 2.0 moles of solute in 200.0 mL of solution. What is its molarity?

2.0 mol

200 mL

1000 mL

1 L= 10.0 mol/L

2. How many grams of Ca(OH)2 are required to make 1.00 L of a 0.750M Ca(OH)2 solution?

74.10 g Ca(OH)21 mol Ca(OH)21 L

= 55.6 g Ca(OH)2

mm Ca(OH)2 = 74.10 g Ca(OH)2 / mol

0.750 mol Ca(OH)2

That is, 55.6 g Ca(OH)2 per liter of solution is 0.750M Ca(OH)2

3. How many grams of CO2 are in 15.0 L of CO2 at STP?  

15.0 L of CO2

44.01 g CO2mol CO222.4 L

= 29.5 g CO2

1.00 mol

Could you use PV = nRT to solve this?  

Why didn’t we use it?

4. 50 mL of an H3PO4 solution was titrated with 0.50 M KOH. 235 mL of the base were required to neutralize the sample. What is the molarity of this acid solution?  

H3PO4 + KOH K3PO4 + H2O

Is this equation balanced?

Why do we need a balanced chemical equation?

4. 50 mL of an H3PO4 solution was titrated with 0.50 M KOH. 235 mL of the base were required to neutralize the sample. What is the molarity of this acid solution?  

H3PO4 + 3 KOH K3PO4 + 3 H2O

0.50 mol KOH0.235

L 3 mol KOH

1 mol H3PO4

0.050 L

= 3.92 10-2 mol H3PO4

3.92 10-2 mol H3PO4

= 0.783 mol/L

1 L

5. What volume of conc hydrochloric acid (12.0 M) is required to make 350 mL of 1.5 M hydrochloric acid?  

Mi Vi = Mf Vf

12.0 M Vi = 1.5 M 350 mL

Vi =12.0 M

1.5 M 350 mL= 43.75 = 44 mL

6. 0.750 mol NH3 occupies 2.25 L at 175ºC. Calculate the pressure of NH3 in the flask.  

PV = nRT

P =

175ºC = 448 K

V

nRT =

0.750 mol 0.0821 L atm mol -1 K-1 448 K

2.25 L

P = 12.3 atm

7. A sample of a gas occupies 2.50103 mL at 25ºC and 560 mmHg. What volume will it occupy at the same temperature and 690 mmHg?  

P1V1 = P2V2

560 mmHg 2.50 103 mL V2

= 690 mmHg

0.737 atm 2.50 103 mL V2

= 0.908 atm

= 2.03 103 mL

= 2.03 103 mL

8. Calculate the mass of 3.54 L of CO gas measured at 29ºC and 800 mmHg.  

29ºC = 302K

PV = nRT

n = RT

PV =

0.0821 L atm mol -1 K-1 302 K

1.05 atm 3.54 L

n = 0.150 mol CO

28.01 g CO

1 mol CO= 4.20 g CO

9. Calculate the amount of heat necessary to raise the temperature of 26.0 g of water from 13.4ºC to 81.0ºC. The specific heat of water = 4.18 J/g·ºC.  

q = msΔt Δt = tf - ti

q = 26.0 g 4.18 J/g ºC (67.6 ºC)

q = 7.35 103 kJ

10. A sample of oxygen gas was collected over water at 25ºC and 725 mmHg. The volume of the container was 4.80 L. Calculate the mass of O2(g) collected.  PT = Pox + Pw

Pox = PT - Pw

Pox = 725 mmHg – 23.76 mmHg

n = RT

PV =

0.0821 L atm mol -1 K-1 298 K

0.923 atm 4.80 L = 18.1 mol

n = 18.1 mol O2

32.00 g O2

1 mol O2

= 579 g O2

Pox = 701.24mmHg = 0.923 atm

11. Calculate the amount of heat required to raise the temperature of 41.7 g of Al from 20.0ºC to 145ºC. (Al has a specific heat of 0.920 J/g·ºC.)  

q = msΔt Δt = tf - ti

q = 41.7 g 0.920 J/g ºC (125 ºC)

q = 4.80 103 kJ

12. Determine the molar mass of an unknown gas if a sample weighing 0.211 g is collected in a flask with a volume of 0.117 L at 90ºC. The pressure of the gas is 682 mmHg.  

n = RT

PV =

0.0821 L atm mol -1 K-1 363 K

0.897 atm 0.117 L = 3.52 10-3 mol

3.52 10-3 mol

0.211 g= 59.9 g / mol

PV = nRT

13. Given: 2 SO2(g) + O2(g) 2 SO3(g) Calculate the heat evolved when 88.2 g of SO2(g) is oxidized to SO3(g).

H = –198.2 kJ/mol  

88.2 g SO2 = -273 kJ1 mol SO2

64.07 g SO2 1 mol

-198.2 kJ

Is this -273 kJ/mol? Explain.