Question #97eda

Your goal here is to figure out the number of grams of sodium chloride, your solute, present in #"100 g"# of this solution.

Since you know that your sample contains #"0.5 g"# of sodium chloride in #"20 g"# of water, you can say that the mass of the solution is equal to

#overbrace("0.5 g")^(color(blue)("mass of solute")) + overbrace("20 g")^(color(blue)("mass of solvent")) = overbrace("20.5 g")^(color(blue)("mass of solution"))#

Now, you know that you have #"0.5 g"# of sodium chloride in #"20.5 g"# of solution, which means that #"100 g"# of this solution will contain

#100 color(red)(cancel(color(black)("g solution"))) * overbrace("0.5 g NaCl"/(20.5color(red)(cancel(color(black)("g solution")))))^(color(blue)("known composition")) = "2.439 g NaCl"#

Since a solution's percent concentration by mass, #"m/m %"#, tells you the number of grams of solute present for every #"100 g"# of solution, you can say that your solution will be

#color(darkgreen)(ul(color(black)("% by mass = 2% NaCl")))#

The answer must be rounded to one significant figure, the number of sig figs you have for your values.