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

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)|)))#