Calculate the molarity of each of the following solutions:
a 10.5 g NaCl in 350.0 mL of solution
b 40.7 g LiClO4 • 3H2
O in 125 mL of solution
22. The concentrated sulfuric acid we use in the laboratory is 98.0% H2
SO4
, by mass.
Calculate the molality and molarity of the acid solution. The density of the solution is
1.83 g/mL.�
a) 10.5 g NaCl = 0.177 mol NaCl
350.0 mL of solution = 0.350 L of solution
Molarity = moles of solute / liters of solution
Molarity = 0.177 mol / 0.350 L = 0.505 M
b) 40.7 g LiClO4 • 3H2O = 0.146 mol LiClO4 • 3H2O
125 mL of solution = 0.125 L of solution
Molarity = moles of solute / liters of solution
Molarity = 0.146 mol / 0.125 L = 1.17 M
22. Since we know the percent concentration by mass and the density, we can calculate the mass of H2SO4 in 1 L of solution:
1 L of solution = 1.83 kg of solution
98.0 g of H2SO4 / 100 g of solution = x g of H2SO4 / 1.83 kg of solution
x = 1.80 kg of H2SO4
We can now use the mass of H2SO4 and the molecular weight to calculate the number of moles:
molecular weight of H2SO4 = 98.08 g/mol
moles of H2SO4 = 1.80 kg / 98.08 g/mol = 18.36 mol
a) Molality is defined as the number of moles of solute per kilogram of solvent. In this case, the solvent is the solution itself (since the acid is dissolved in water), so we need to calculate the mass of solvent:
mass of solvent = 1.83 kg - 1.80 kg = 0.03 kg
molality = moles of solute / kilograms of solvent
molality = 18.36 mol / 0.03 kg = 612 mol/kg
b) Molarity is defined as the number of moles of solute per liter of solution. We already calculated the number of moles of H2SO4 in 1 L of solution, so we can use that:
Molarity = moles of solute / liters of solution
Molarity = 18.36 mol / 1 L = 18.36 M