# Question #cb299

##### 1 Answer

#### Explanation:

Don't forget that a solution's **percent concentration by mass** tells you the amount of solute present **for every** **of solution**.

In other words, the solution's percent concentration by mass is a measure of the number of grams of solute present for every **solute and solvent**.

You can replace *grams* with *kilograms* if you want and say that a **by mass** sodium chloride solution will contain **for every**

#"40 kg NaCl " stackrel(color(white)(acolor(red)("for every")aaa))(->) "100 kg NaCl" + "H"_2"O"#

You already know that you have *kilograms* of sodium chloride that must be added to

You know that

#(3 + x)color(white)(.)"kg solution"#

will be equivalent to

#(xcolor(red)(cancel(color(black)("kg NaCl"))))/((3 + x) color(red)(cancel(color(black)("kg solution"))) ) = (40 color(red)(cancel(color(black)("kg NaCl"))))/(100color(red)(cancel(color(black)("kg solution"))))#

This means that you will have

#x = 40/100 (3 + x)#

#100x = 120 + 40x#

#60x = 120 implies x= 120/60 = 2#

Therefore, you need to add **by mass** sodium chloride solution.