Calculate Molarity and Concentration

Question

1. Calculate the molarity of the following solutions:

a. A sample of solid KBr weighing 8.90 g is put in enough distilled water to give 200.0 mL of

solution.

b. A 24.9 g sample of NaCl is dissolved in a quantity of water, and the solution is stirred well. A

500.0 mL of that solution was prepared.

2. Calculate the concentration in %(w/w) of the following solutions. Assume water has a density of 1.00

g/mL.

a. 0.200 mol HCl is dissolved in 150.0 mL of water.

b. 5.20 g of CaCl2 is dissolved in 250 mL of water.

Answer 1

Molarity is defined as moles of solute per liter of solution.

a) molar mass KBr = 119 g/mol

mols KBr = 8.90 g x 1 mol / 119 g = 0.07479 mols

volume = 200.0 ml x 1 L/1000 ml = 0.2000 L

Molarity = 0.07479 mols / 0.2000 L = 0.374 M (3 sig. figs.)

b) molar mass NaCl = 58.4 g / mole

Do the calculations the same as in (a) above

2) % w/w means grams of solute per 100 grams of solution

a) molar mass HCl = 36.5 g / mol

0.200 mol x 36.5 g / mol = 7.3 g HCl

150.0 ml H2O x 1 g / ml = 150 g H2O

Total mass of solution = 7.3 g + 150. g = 157.3 g

% (w/w) = 7.3 g / 157.4 g (x100%) = 4.64%

b) do the calculations the same as is 2(a) above