How many more grams of KI will dissolve in 100 g of water at 40 °C than at 20°C?

Your tool of choice here will be the solubility graph for potassium iodide, `"KI"`, which looks like this

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Now, this solubility graph expresses the solubility of potassium iodide per `"100 cm"^3` of water, but you can use it as a measure of the salt's solubility per `"100 g"` of water.

Notice that at `40^@"C"`, a saturated potassium iodide solution will contain about `"195 g"` of dissolved potassium iodide.

At `20^@"C"`, on the other hand, a saturated potassium iodide solution will only contain about `"155 g"` of dissovled potassium iodide.

These values can be found by tracing vertical lines starting from `40^@"C"` and `20^@"C"`, respectively, until they intersect the solubility curve for potassium iodide. The values that correspond to these points represent the solubility of the salt in a saturated solution.

Therefore, you can say that a saturated solution of potassium iodide will hold `"40 g"` more potassium iodide at `40^@"C"` than at `20^@"C"`.

That is the case because

`overbrace("195 g")^(color(blue)("solubility at 40"^@"C")) - overbrace("155 g")^(color(purple)("solubility at 20"^@"C")) = "40 g"`