# Question 68fe6

Jul 4, 2016

$\text{0.092 g}$

#### Explanation:

The problem wants you to use the number of moles, $n$, present in a sample of a given compound and the molar mass, $M$, of said compound to determine the mass of the sample, $m$.

The idea here is that a compound's molar mass can be used as a conversion factor between number of moles and mass expressed in grams.

That is the case because the molar mass essentially tells you the mass of one mole of a given compound. In your case, the compound is said to have a molar mass of

$M = \text{44 g/mol}$

This tells you that every mole of said compound has a mass of $\text{44 g}$.

Since you know that the sample contains $0.0021$ moles, you can say that its mass will be equal to

0.0021 color(red)(cancel(color(black)("moles"))) * overbrace("44 g"/(1color(red)(cancel(color(black)("mole")))))^(color(purple)("= molar mass")) = color(green)(|bar(ul(color(white)(a/a)color(black)("0.092 g")color(white)(a/a)|))) -> rounded to two sig figs.

This can also be calculated by using the equation

$\textcolor{b l u e}{| \overline{\underline{\textcolor{w h i t e}{\frac{a}{a}} m = n \cdot {M}_{M} \textcolor{w h i t e}{\frac{a}{a}} |}}}$

Since the mass of the sample is equal to the number of moles it contains multiplied by the molar mass of the compound, you will once again have

m = 0.0021 color(red)(cancel(color(black)("moles"))) * "44 g"/color(red)(cancel(color(black)("mol"))) = color(green)(|bar(ul(color(white)(a/a)color(black)("0.092 g")color(white)(a/a)|)))#

The answer must be rounded to two sig figs because that's how many sig figs you have for the number of moles present in the sample.