# Question #1cb75

##### 1 Answer

He was a little careless with significant figures at that point.

The calculation should have been

The mass of O₂ determines the number of significant figures.

The calculation for

You use all the digits in the moles of O₂, but you must round the answer to two significant figures, because the volumes have only two significant figures.

When you calculate the mass of O₂, you again use all the digits that your calculator gave for the moles. Then you round off the final answer to two significant figures.

Mass of O₂ =

He multiplied by the rounded number of moles and got 83 g of O₂. This is called *round-off error*.

Hope this helps.