Question

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?

Answer 1

`"32 g KNO"_3`

Explanation:

Your tool of choice here will be the solubility graph for potassium nitrate, `"KNO"_3`, which looks like this

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

You know that at this temperature, your solution contains `"35 g"` of potassium nitrate in `"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 `"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 `"32 g"` of potassium nitrate in your solution at `40^@"C"` will get you a saturated solution. Any extra amount of potassium nitrate that you will add past this point will remain undissolved.