Answer: Oxygen
Explanation:
As seen from the balanced equation:
2 moles of hydrogen combine with 1 mole of oxygen or 1 mole of hydrogen combine with 1/2 mole of oxygen.
According to Avogadro's law, equal volumes of gases contain equal number of molecules and hence equal number of moles.
Thus if x moles of both gases are present, x moles of hydrogen will combine with 
moles of oxygen and 
moles of oxygen will be left used and will be in excess.
The electrons hold onto the nucleus because of the magnetism and the electricity. <span />
If enough HCl is not added, the percent yield will be lower as all the magnesium metal has not reacted with HCl.
<h3>What is percent yield of a reaction?</h3>
The percent yield of a reaction is the ratio of the actual yield and the expected yield expressed as a percentage.
- Percent yield = actual yield/expected yield × 100%
The reaction between magnesium metal and HCl liberates hydrogen gas and forms magnesium chloride salt.
If enough HCl was not added to react with all the magnesium metal, the actual yield will be lower.
Hence, percent yield will be lower as well.
Therefore, the percent yield will be lower if all the magnesium metal has not reacted with HCl.
Learn more about percent yield at: brainly.com/question/10046114
