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?

1 Answer
Jun 24, 2016

Answer:

#"32 g KNO"_3#

Explanation:

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

http://www.mts.net/~alou/Chemistry%2011/Unit%204%20-%20Solutions%20Lessons

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.