# Suppose that 7.47 x 10^-3mol of hydrogen gas occupies a 335 mL container at -91°C what is the pressure (in torr)? i know the answer is 253 .. but idk how to work it? someone please help me?

Oct 21, 2015

Use the ideal gas law equation to find the pressure of the sample.

#### Explanation:

You know that your sample of hydrogen occupies a volume of $\text{3.35 mL}$ at a temperature of $- {91}^{\circ} \text{C}$, and are interested in finding what the pressure of the sample is.

Since you know number of moles, volume, and temperature, you can use the ideal gas law equation to determine the pressure of the sample.

$P V = n R T \text{ }$, where

$P$ - the pressure of the hydrogen sample
$V$ - the volume it occupies
$n$ - the number of moles of gas;
$R$ - the universal gas constant, usually given as $0.082 \left(\text{atm" * "L")/("mol" * "K}\right)$
$T$ - the temperature of the gas - expessed in Kelvin

So, rearrange the ideal gas law equation to solve for $P$, the pressure of the gas.

$P = \frac{n R T}{V}$

Plug in your values and find $P$ - do not forget that you need to use the volume in liters and the temperature in Kelvin

$P = \left(7.47 \cdot {10}^{- 3} \textcolor{red}{\cancel{\textcolor{b l a c k}{\text{moles"))) * 0.082("atm" * color(red)(cancel(color(black)("L"))))/(color(red)(cancel(color(black)("mol"))) * color(red)(cancel(color(black)("K")))) * (273.15 - 91)color(red)(cancel(color(black)("K"))))/(335 * 10^(-3)color(red)(cancel(color(black)("L}}}}\right)$

$P = \text{0.33306 atm}$

To get the pressure in torr, you need to use the conversion factor

$\text{1 atm " = " 760 torr}$

This will get you

0.33306color(red)(cancel(color(black)("atm"))) * "760 torr"/(1color(red)(cancel(color(black)("atm")))) = color(green)("253 torr")

Here are some cool videos to help you get comfortable with the ideal gas law equation