Question

How do you find the number of moles for a gas from temperature and pressure?

Answer 1

You use the Ideal Gas Law.

Explanation:

You also need the volume.

Then, you insert the pressure, volume, and temperature into the expression and calculate the number of moles.

The Ideal Gas Law is

`color(blue)(bar(ul(|color(white)(a/a)pV = nRTcolor(white)(a/a)|)))" "`

We can rearrange this formula to get the number of moles, `n`.

`n = (pV)/(RT)`

EXAMPLE

A 3.00 L container is filled with neon gas at 770 mmHg and 27 °C. How many moles of neon are present?

Solution

Step 1. List the information given, making conversions as necessary.

`p = 770 color(red)(cancel(color(black)("mmHg"))) × "1 atm"/(760 color(red)(cancel(color(black)("mmHg")))) = "1.013 atm"`
`V = "3.00 L"`
`R = "0.082 06 L·atm·K"^"-1""mol"^"-1"`
`T = "(30 + 273.15) K = 303.15 K"`

Step 2. Calculate the number of moles.

`n = (1.013 color(red)(cancel(color(black)("atm"))) × 3.00 color(red)(cancel(color(black)("L"))))/("0.082 06" color(red)(cancel(color(black)("L·atm·K"^"-1")))"mol"^"-1" × 300.15 color(red)(cancel(color(black)("K")))) = "0.123 mol"`

There are 0.123 mol of neon in the container.