Question

You have 40g of sodium chloride. What is the minimum amount of water required to dissolve the salt at 30oC?

Answer 1

The minimum mass of water is 111 g.

Explanation:

Step 1. Determine the solubility of `"NaCl"` at 30 °C

I am guessing that "they" are asking you to get the information from a solubility curve for sodium chloride like the one below.

(Adapted from http://www.sciencegeek.net/Chemistry/taters/solubility)

From 30 °C on the horizontal axis, draw a vertical line up until it hits the solubility curve (blue) for `"NaCl"`.

Then draw a horizontal line to the left until it meets the vertical axis, and read the solubility.

The solubility is `"36 g NaCl/100 g water"`.

Step 2. Calculate the mass of water required

`"Mass of water" = 40 color(red)(cancel(color(black)("g NaCl"))) × "100 g water"/(36 color(red)(cancel(color(black)("g NaCl")))) = "111 g water"`