Question

What is the numerical setup for calculating the number of moles of `CO_2` present in 11 grams of `CO_2`?

Answer 1

Here's how to get the numerical setup.

Explanation:

You must use the molar mass of `"CO"_2` as a conversion factor to convert grams of `"CO"_2` to moles of `"CO"_2`.

The molar mass of `"CO"_2` is 44.01 g/mol.

Thus, the conversion is either (a) `("44.01 g CO"_2)/("1 mol CO"_2)` or (b) `("1 mol CO"_2)/("44.01 g CO"_2)`.

You want an equation like

`"11 g CO"_2 × "conversion factor" = "? mol CO"_2`

The conversion factor must have the desired units of your answer (`"mol CO"_2`) on top, so you choose option (b).

The numerical setup is

`11 color(red)(cancel(color(black)("g CO"_2))) × ("1 mol CO"_2)/(44.01 color(red)(cancel(color(black)("g CO"_2))))= "0.25 mol CO"_2`

Note that the units of "`"g CO"_2`" cancel and give you the correct units for the answer.