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 2.800×10−2 m solution of nacl in water is at 20.0∘c. the sample was created by dissolving a sample of nacl in water and then bringing the volume up to 1.000 l. it was determined that the volume of water needed to do this was 999.2 ml . the density of water at 20.0∘c is 0.9982 g/ml.

Answer 1

Answer: -

Molality of NaCl = 2.807 ×10⁻² m

Explanation: -

Molarity given = 2.800×10⁻² M

This means there are 2.800×10⁻² moles of NaCl per 1000 mL of solution.

Volume of water = 999.2 ml

Density of water = 0.9982 g/ml

Mass of water = Density of water x Volume of water

                       = 0.9982 g/ml x 999.2 ml

                        = 997.4 g

Thus 997.4 g of water has 2.800×10⁻² moles of NaCl.

1000 g of water has $\frac{1000}{997.4}$ x 2.800×10⁻² moles of NaCl.

                                = 2.807 ×10⁻² moles of NaCl.

Molality of NaCl = 2.807 ×10⁻² m