Question

Question #10db8

Answer 1

`2.8 * 10^8` `"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.

`"density" = "mass of substance"/"1 unit of volume"`

In your case, the density of helium at STP is equal to `"0.1875 g L"^(-1)`, which means that under STP conditions, every `"1 L"` of helium has a mass `"0.1875 g"`.

`"0.1875 g L"^(-1) = "0.1875 g"/"1 L"`

Now, you know that the volume of the balloon is equal to `1.5 * 10^9` `"L"` under STP conditions.

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

`1.5 * 10^9 color(red)(cancel(color(black)("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.