A sample of mercury with a mass of 114.0g was combined with 12.8g of oxygen gas, and the resulting reaction gave 123.1g of mercury(II) oxide.

How much oxygen was left over after the reaction was complete?

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The reaction can be written as,

2Hg + O2 --> 2HgO

For the calculation the following molecular weights are considered.

Hg - 200.6 g/mol, O - 16 g/mol

according to the stoichiometric reaction, 401.2 g of Hg reacts with 32 g of O2 and produces 433.2 g of HgO

Hence the amount of O2 that will react with 114.0 g of Hg can be calculated as,

=> (114/401.2)x32

=> 9.1 g

Therefore the amount of excess O2 is = 12.8 g - 9.1 g

= **3.7 g**

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