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.
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