Question

What is the maximum number of grams of `"NH"_4"Cl"` that will dissolve in 200 grams of water at 70°C?

Answer 1

`"120 g NH"_4"Cl"`

Explanation:

Your tool of choice here will be the solubility graph for ammonium chloride, `"NH"_4"Cl"`, which looks like this

The solubility graph provides you with information on the solubility of a given salt at various temperatures. In this case, the graph shows you the solubility of ammonium chloride per `"100 g"` of water at temperatures ranging from `0^@"C"` to `100^@"C"`.

The actual curve represents the solubility of the salt, i.e. the maximum amount that can be dissolved in `"100 g"` of water to form a saturated solution.

At `70^@"C"`, ammonium chloride has a solubility of about `"62 g / 100 g H"_2"O"`. This tells you that can only hope to dissolve `"62 g"` of ammonium chloride for every `"100 g"` of water before the solution becomes saturated.

After this point, any extra salt added would remain undissolved.

So, you know how much salt can be dissolved in `"100 g"` of water, so use this as a conversion factor to find the solubility of the salt in `"200 g"` of water

`200 color(red)(cancel(color(black)("g H"_2"O"))) * ("62 g NH"_4"Cl")/(100color(red)(cancel(color(black)("g H"_2"O")))) = "124 g NH"_4"Cl"`

I will leave the answer rounded to two sig figs, but keep in mind that you only have one sig fig for the mass of water

`"solubility at 70"""^@"C in 200 g H"_2"O" = color(green)(|bar(ul(color(white)(a/a)color(black)("120 g")color(white)(a/a)|)))`