Question #07095

A solution's percent concentration by mass, or #"% m/m"#, essentially tells you how many grams of solute you get for every #"100 g"# of solution.

In your case, an ammonia solution that is #"12.5% m/m"# ammonia, #"NH"_3#, will contain #"12.5 g"# of ammonia, which is the solute, for every #"100 g"# of solution.

This means that you can essentially use the solution's percent concentration as a conversion factor to help you go from grams of solute to grams of solution or vice versa.

In your case, #"135 g"# of this ammonia solution will contain

#135 color(red)(cancel(color(black)("g solution"))) * overbrace("12.5 g NH"_3/(100color(red)(cancel(color(black)("g solution")))))^(color(purple)("= 12.5% m/m")) = color(green)(|bar(ul(color(white)(a/a)"16.9 g NH"_3color(white)(a/a)|)))#

The answer is rounded to three sig figs.

Now, you can use the exact same approach to find the volume of fruit juice present in #"250 mL"# of a #"15% v/v"# solution.

This time, you're dealing with percent concentration by volume, or #"% v/v"#, which essentially tells you the volume of solute that you get for every #"100 mL"# of solution.

Use this concentration as a conversion factor to get the answer. You should end up with #"37.5 mL"#.