How many grams of #"KNO"_3# should be dissolved in water to make #"500.0 g"# of a #"20.0-ppm"# solution?

1) 1.00 x 10^-3 g
2) 1.00 x 10^-4 g
3) 1.00 x 10^-1 g
4) 1.00 x 10^-2 g

1 Answer
Feb 28, 2018

Answer:

#1.00 * 10^(-2)# #"g"#

Explanation:

As you know, a solution's concentration in parts per million tells you the number of grams of solute present for every

#10^6 = 1,000,000#

grams of the solution. In your case, the target solution must have a concentration of #"20.0 ppm"#, which implies that it must contain #"20.0 g"# of potassium nitrate, the solute, for every #10^6# #"g"# of the solution.

This means that the mass of potassium nitrate needed to make this solution is equal to

#500.0 color(red)(cancel(color(black)("g solution"))) * "20.0 g KNO"_3/(10^6color(red)(cancel(color(black)("g solution")))) = color(darkgreen)(ul(color(black)(1.00 * 10^(-2) quad "g")))#

The answer is rounded to three sig figs.

So, if you dissolve #1.00 * 10^(-2)# #"g"# of potassium nitrate in enough water to get a total mass of the solution to #"500.0 g"#, you will have a #"20.0-ppm"# potassium nitrate solution.