# A solution contains 35 grams of KNO_3 dissolved in 100 grams of water at 40°C. How much more KNO_3 would have to be added to make it a saturated solution?

Jun 24, 2016

${\text{32 g KNO}}_{3}$

#### Explanation:

Your tool of choice here will be the solubility graph for potassium nitrate, ${\text{KNO}}_{3}$, which looks like this

As you can see, potassium nitrate has s solubility of about $\text{67 g / 100g H"_2"O}$ at ${40}^{\circ} \text{C}$. This means that at ${40}^{\circ} \text{C}$, a saturated potassium nitrate solution will contain $\text{67 g}$ of dissolved salt for every $\text{10 0g}$ of water.

You know that at this temperature, your solution contains $\text{35 g}$ of potassium nitrate in $\text{100 g}$ of water. This solution will be unsaturated because it contains less potassium nitrate than the maximum amount that can be dissolved.

In order to make a saturated solution, you must get the total mass of potassium nitrate to $\text{67 g}$, which means that you must add

"mass of KNO"_3 color(white)(a)"to be added" = "67 g" - "35 g" = color(green)(|bar(ul(color(white)(a/a)color(black)("32 g")color(white)(a/a)|)))

So, dissolving another $\text{32 g}$ of potassium nitrate in your solution at ${40}^{\circ} \text{C}$ will get you a saturated solution. Any extra amount of potassium nitrate that you will add past this point will remain undissolved.