# Question c5ce9

Apr 7, 2016

${\text{2 mol L}}^{- 1}$

#### Explanation:

Molar concentration essentially depends on two things

• how many moles of solute you have present
• the total volume of the solution, expressed in liters

You're dealing with a sample of hydrogen gas, ${\text{H}}_{2}$, so you're looking for the volume of the reaction vessel, not of the solution.

The problem provides you with the mass of hydrogen gas, so use its molar mass as a conversion factor to go figure out how many moles would be present here

20 color(red)(cancel(color(black)("g"))) * "1 mole H"_2/(2.016color(red)(cancel(color(black)("g")))) = "9.921 moles H"_2

Now, in order to find the molar concentration of the gas, you must figure out how many moles would occupy a volume of one liter. To do that, use the given fact that $9.921$ moles occupy $\text{5 L}$

1 color(red)(cancel(color(black)("L"))) * "9.921 moles H"_2/(5color(red)(cancel(color(black)("L")))) = "1.984 moles H"_2

So, if one liter contains $1.984$ moles of hydrogen gas, it follows that its molar concentration is ${\text{1.984 mol L}}^{- 1}$.

Rounded to one significant figure, the answer will be

["H"_2] = color(green)(|bar(ul(color(white)(a/a)"2 mol L"^(-1)color(white)(a/a)|)))#