# Question #5498c

Oct 22, 2017

$\text{428 mL}$

#### Explanation:

As you know, the *molarity of a solution tells you the number of moles of solute present for every $\text{1 L}$ of the solution.

$\text{molarity" = "moles"/"1 L}$

If you keep in mind that you have

$\text{1 mol" = 10^3color(white)(.)"mmol" " }$ and $\text{ " "1 L" = 10^3color(white)(.)"mL}$

you can say that the molarity of a solution tells you the number of millimoles of solute present for every $\text{1 mL}$ of the solution.

$\text{molarity" = (color(red)(cancel(color(black)(10^3)))color(white)(.)"mmoles")/(color(red)(cancel(color(black)(10^3)))color(white)(.)"mL") = "mmoles"/"1 mL}$

This means your solution, which has a molarity of $\text{0.642 M}$, contains $0.642$ mmoles of sodium thiosulfate, the solute, for every $\text{1 mL}$ of the solution.

You can thus say that $275.$ mmoles of sodium thiosulfate will be present in

$275. \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{mmoles Na"_2"S"_2"O"_3))) * "1 mL solution"/(0.642 color(red)(cancel(color(black)("mmoles Na"_2"S"_2"O"_3)))) = color(darkgreen)(ul(color(black)("428 mL solution}}}}$

The answer is rounded to three sig figs.