In: Chemistry
Chemists often use molarity M, in moles/liter, to measure the concentration of solutions. Molarity is a common unit of concentration because the volume of a liquid is very easy to measure. However, the drawback of using molarity is that volume is a temperature-dependent quantity. As temperature changes, density changes, which affects volume. Volume markings for most laboratory glassware are calibrated for room temperature, about 20?C.
Fortunately, there are several other ways of expressing concentration that do not involve volume and are therefore temperature independent.
A 2.350
a) Molality = mol of solute / mass of solvent.
Mol of solute = Molarity of solute x Volume of solution = (0.02350 M) x (1.000 L) = 0.02350 mol of solute
Mass of solvent = 999.4 mL x (0.9982 g / 1 mL) = 1001.202 g = 1.0012 kg
Molality = 0.02350 mol / 1.0012 kg
Molality= 0.02347mol / kg =2.347*10^-2mol / kg
b) Mol fraction of salt = mol of salt / mol
total
Mol of water = 997.7 g x (1 mol H2O / 18.0154 g H2) = 55.38 mol H2O
Mol fraction of salt = 0.02350 mol / (0.02350 mol +
55.38 mol) = 4.2416*10^-4
c) Concentration by percent mass = (mass of salt / mass of
solution) x 100%
Mass of salt = 0.02350 mol NaCl x (58.44 g NaCl / 1 mol NaCl) = 1.37334 g NaCl
Mass of water = 997.7 g H2O
Mass of solution = 1.37334 g + 997.7 g = 999.07334g
Therefore, (1.37334 g / 999.07334 g) x 100% = 0.13746 %
by mass
d) Parts per million = (g of solute / g of
solution)*1000000
= (1.37334 g / 999.07334 g)*1000000 = 1374.6138 ppm