Question f0918

Jun 12, 2016

6.3%

Explanation:

The trick here is to realize that a solution's weight by weight percent concentration, $\text{%w/w}$, is calculated by taking into account the mass of the solute, which in your case is potassium nitrate, ${\text{KNO}}_{3}$, and the total mass of the solution.

You will thus have

color(blue)(|bar(ul(color(blue)("% w/w KNO"_3 = "mass of KNO"_3/"total mass of solution" xx 100color(white)(a/a)|)))

You know that your solution contains $\text{8.4 g}$ of potassium nitrate and $\text{125 g}$ of water. This means that the total mass of the solution will be

${m}_{\text{total}} = {m}_{K N {O}_{3}} + {m}_{{H}_{2} O}$

${m}_{\text{Total" = "8.4 g" + "125 g" = "133.4 g}}$

The weight by weight percent concentration will thus be

"% w/w" = (8.4 color(red)(cancel(color(black)("g"))))/(133.4color(red)(cancel(color(black)("g")))) xx 100 = color(green)(|bar(ul(color(white)(a/a)color(black)("6.3% KNO"_3)color(white)(a/a)|)))#

This means that for every $\text{100 g}$ of solution you get $\text{6.3 g}$ of potassium nitrate.