Getz 2007

Practice Problems, Part 1: Molarity, molality, Normality

Solve each problem on your own paper. Be sure to show:

• Information the problem gives you- numbers and its description

• Set up of the math

• Doing the math; use dimensional analysis whenever possible

• Answer before rounding

• Answer after rounding

• No naked numbers- put units on all numbers

1. Calculate the Molarity of a solution that has 2.3 moles dissolved in 4.6

liters total.

2.3 moles

4.6 liters of solution

Need to solve for Molarity

Moles = ?

Liters

2.3 moles = 0.50 moles/L = 0.50M

4.6 liters

2. Calculate the molality of a solution that has 1.5 moles added to 675 mL of

solvent.

1.5 moles

675 mL of solvent

Need to convert mL to L

Need to solve moles/L

675 mL x 1L = 0.675 L

1000mL

1.5 moles = 2.2222 moles =2.2 moles/liter = 2.2 m

0.675 L 1 L of solvent

3. Calculate the Molarity of 23.5 grams of silver nitrate, AgNO3, that is

dissolved in water for a final volume of 750 mL.

23.5 g AgNO3

750 mL final volume

Need to convert g to moles

Need to convert 750 mL to L

Getz 2007

Need to use equation moles/liter

Ag = 107.9 g/mol

N = 14.0 g/mol

3 oxygens = 3 (16.0) = 48.0 g/mol

107.9 + 14.0 + 48.0 = 169.9 g/mol

23.5 grams x 1 mol = 0.1383 mol = 0.138 mol

169.9 g

750 mL x 1 L = 0.75 L

1000 mL

0.138 mol = 0.1844 mol/L = 0.18 mol/L = 0.18 M

0.75 L

4. How many grams of CuSO4 5H2O do you need to make 800. mL of a 0.6 M

solution? Remember to calculate the molar mass of this chemical you figure

out the mass of Cu, S, four oxygens, and five waters. You treat the as a +

not as an x.

800. mL

0.6 M

Need grams of CuSO4 5H2O

Need to figure out mol of CuSO4 5H2O then how many grams it is.

Convert 800. mL to L

800.mL x 1 L = 0.8 L

1000mL

0.8 L x 0.6 mol = 0.48 mol

1 L

Cu = 63.5 g/mol

S = 32.0 g/mol

O = 16.0 g/mol so 4- O = 64.0 g/mol

H2O = 18.0 g/mol so 5 H2O = 90.0 g/mol

Molecular weight of copper (II) sulfate =

0.48 mol x 249.5 g = 119.76 g

1 mol

Getz 2007

5. You have 20.0 grams of Pb(NO3)2. You put it in water and bring the final

volume to 50.0 mL. What is the Molarity?

20.0 grams

50.0 mL

Need to convert g to mol

Need to convert 50.0 mL

Need to use equation moles/liter

Pb = 207.1 g/mol

2 N = 2(14) = 28.0 g/mol

6- O = 6(16)= 96.0 g/mol

207. 1 + 28.0 + 96.0 = 331.1 g/mol

20.0 g x 1 mol = 0.0604 mol

331.1 g

50.0 mL x 1 L = 0.050 L

1000mL

0.0604 mol = 1.208 mol/L = 1.2 mol/L = 1.2 M

0.050 L

6. What is the normality of a 2 M solution of H3PO4?

2 moles of H3PO4 x 3 moles H+ = 6 moles H+ so 6N

1 liter 1 mole of H3PO4 1 liter

7. How many grams of NaOH are in 400.0 mL of a 0.5 M solution of NaOH?

400.0 mL

0.5 M

Need to convert 400.0 mL to L

Need to figure out molecular weight of NaOH

400.0 mL x 1 L = 0.4000 L

1000mL

0.4000 L x 0.5 mol = 0.2 mol

1 L

Getz 2007

Na = 23.0 g/mol

O= 16.0 g/mol

H = 1.0 g/mol

NaOH = 40.0 g/mol

0.2 mol x 40.0 g = 8.0 g

1 mol

8. We need 850.0 mL of a 0.45 M solution of NaHCO3. How much NaHCO3 do

we weigh?

850.0 mL

0.45 M

Convert 850.0 mL to L

Need to figure out mol

Need to figure out molecular weight of NaHCO3 to get to g of NaHCO3

850.0 mL x 1 L = 0.8500L

1000mL

0.8500 L x 0.45 moles = 0.3825moles

1 L

Na= 23.0 g/mol

H= 1.0 g/mol

C = 12.0 g/mol

3- O = 48.0 g/mol

NaHCO3= 84.0 g/mol

0.3825 moles x 84.0 g = 32.13 g = 32.1 g or 32 g

1 mol

9. Which solution will have a greater volume?

a. 58.5 g of NaCl in water to make 1000 mL of solution

b. 58.5 g of NaCl added to 1000 mL of water

b will have a greater volume, even though it will not be that much greater

Getz 2007

10. Identify the descriptions in problem 9a and 9b as being either Molarity

or molality.

a. Molarity

b. molality

11. Calculate the Molarity or the molality for the descriptions in 9a and 9b.

You need to do question 10 first; figure out which one is a description of

Molarity and which is molality first. Make sure you use the proper units.

NaCl = 58.5 g/mol

So for 9a it would be 1 M and for 9b it would be 1 m

58.5 g x 1 mol = 1 mol

58.5 g