# Question #10db8

Jun 20, 2017

$2.8 \cdot {10}^{8}$ $\text{g}$

#### Explanation:

The thing to remember about the density of a substance is that it represents the mass of exactly one unit of volume of said substance.

$\text{density" = "mass of substance"/"1 unit of volume}$

In your case, the density of helium at STP is equal to ${\text{0.1875 g L}}^{- 1}$, which means that under STP conditions, every $\text{1 L}$ of helium has a mass $\text{0.1875 g}$.

$\text{0.1875 g L"^(-1) = "0.1875 g"/"1 L}$

Now, you know that the volume of the balloon is equal to $1.5 \cdot {10}^{9}$ $\text{L}$ under STP conditions.

Since you already know that you get $\text{0.1875 g}$ of helium per $\text{1 L}$, use the density of the helium at STP as a conversion factor to find the mass present in your balloon

$1.5 \cdot {10}^{9} \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{L"))) * overbrace("0.1875 g"/(1color(red)(cancel(color(black)("L")))))^(color(blue)("the density of helium at STP")) =color(darkgreen)(ul(color(black)(2.8 * 10^8color(white)(.)"g}}}}$

The answer is rounded to two sig figs, the number of sig figs you have for the volume of the balloon.