# Question #055ce

May 18, 2017

You would have to add 57 g of solute.

#### Explanation:

I assume that your instructor is asking you to use a solubility curve like the one below to answer the question.

Note that the curve gives the solubility in terms of mass of solute per 100 g of water.

It shows that the solubility of potassium nitrate is 49 g/100 g water at 30 °C
and 140 g/100 g water at 70 °C.

You are using only 63 g of water.

At 30 °C,

${\text{Solubility" = 63 color(red)(cancel(color(black)("g water"))) × ("49 g KNO"_3)/(100 color(red)(cancel(color(black)("g water")))) = "30.9 g KNO}}_{3}$

At 70 °C,

${\text{Solubility" = 63 color(red)(cancel(color(black)("g water"))) × ("140 g KNO"_3)/(100 color(red)(cancel(color(black)("g water")))) = "88.2 g KNO}}_{3}$

Thus, to keep the solution saturated at 70 °C, you would have to add

$\text{88.2 g - 30.9 g = 57 g}$ of ${\text{KNO}}_{3}$.

Note: The answer can have no decimal places because we did not read the graph to any decimal places.