Chemists often use molarity M , in moles/liter, to measure the concentration of

Question

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

Explanation / Answer

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

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