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 2.550

Explanation / Answer

molarity = moles / L of solution

so moles of solute = molarity X volume of solution = 2.55 X 10^-2 X 1 = 2.55 X 10^-2 moles

1. molality = moles of solute / Kg of solvent = 2.55 X 10^-2 / 0.997 = 2.557 X 10^-2 molal

2.mole fraction = moles of salt / total moles = 2.55 X 10^-2 / 2.55 X 10^-2 + 55.38 = 2.55 X 10^-2 / 55.405 = 0.046 X 10^-2

3. Percent by mass = mass of salt / total mass of solution

mass of solute = moles X mol wt = 2.55 X 10^-2 X 58.5 = 1.49g

mass of water = 997g

so mass % = 1.49 X 100 / 998.49 = 0149%

4. cocentration in ppm = mass of solute in mg / L of solution = 1.49 X 10^3 / 1 = 1490ppm

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