Question #b76e5

Depending on what exactly you're dealing with, you can have

• Determine the number of moles from mass

The molar mass is the most important tool you have at your disposal when tyring to determine number of moles present in a sample.

The molar mass of a substance tells you what the mass of 1 mole of that substance is. This means that you can use it to convert between mass and moles and vice versa.

`"molar mass" = "mass"/"1 mole"`

So, if you know the mass of a substance, you acn use its molar mass to determine how many moles would be present in the sample.

`"X"cancel("grams") * "1 mole"/(Ycancel("g")) = "X"/"Y" "moles"`

In other words, if you have `"X"` grams of a substance that has a molar mass of `"Y g/mol"`, you're going to have `"X"/"Y"` moles of that substance present.

Likewise, if you know the number of moles you have, you can use the molar mass to get the mass of the sample.

`"Z"cancel("moles") * ("Y g")/(1cancel("mole")) = "ZY g"`

I you have `"Z"` moles of a substance that has a molar mass of `"Y g/mol"`, the sample will have a mass of `"ZY g"`.

• Determine the number of moles from the number of particles

One mole of any substance contains exctly `6.0222 * 10^(23)` atoms or molecules of that substance - this is known as Avogadro's number. SO, if you have, say, `3.011 * 10^(20)` molecules of a substance, you'll have

`12.044 * 10^(22)cancel("molecules") * "1 mole"/(6.022 * 10^(23)cancel("molecules")) = 2 * 10^(-1)"moles"`

• Determine the number of moles of an ideal gas at STP

If you're dealing with gases, you can use the molar volume of a gas at STP to get the number of moles present in a volume of gas.

At STP, 1 mole of any ideal gas occupies exactly 22.4 L. This means that, if you have 22.4 L of a gas, you'll have one mole present.

`"X"cancel("L") * "1 mole"/(22.4cancel("L")) = "X"/22.4"moles"`

If you have a gas that occupies a volume of `"X"` liters at STP, the number of moles of that gas present will be equal to `"X"/22.4`.