Question

A 2.950×10−2 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. 1. Calculate the molality of the salt solution 2. Calculate the mole fraction of salt in this solution 3. Calculate the concentration of the salt solution in percent by mass 4. Calculate the concentration of the salt solution in parts per million.

Answer 1

1. molality = moles of NaCl/kg of water

it says they used 999.2mL of water to create a 1L solution so the other (1000mL - 999.2mL) must have been NaCl. calculate the moles of NaCl using molarity * volume = moles

use the density of water 0.9982 g/mL to calculate the mass, in kg, of 999.2mL water (be careful w/ units)

Answer 2

2. mole fraction = moles of NaCl / (moles of NaCl + moles of water)

you already have moles of NaCl from part 1, and kg of water. so you just need to convert mass of water to moles using the molar mass and it should be straightforward from there

Answer 3

3. percent by mass = 100 * (mass of NaCl)/ (total mass)

you have moles of NaCl, simply multiply by MM to get g
you should have kg of water from part 1, so multiply that by 1000 to get the mass of water in g

should be a straightforward calculation from there

Answer 4

4. i'm not 100% on this but ppm can be mg NaCl/L solution