What is the equation for calculating moles?Mass – Mr Mr/massMass × MrMass/Mr2How many moles are there in 1.2 g of sodium hydroxide?4040.031.23What is the first thing you should do when calculating reacting masses?Calculate the Mr for each substanceCalculate the ratio of the moles for each substanceCalculate the moles for each substanceWrite out the balanced equation4What do we call a reactant that is left over at the end of a reaction?In excessBalancedWastedLimiting5Three moles of Mg react with 5 moles of HCl to form MgCl2 and H2. What is the limiting reagent in this reaction?HClMgH2MgCl2

The text you provided seems to be a set of multiple-choice questions related to chemistry, specifically about moles, reacting masses, and limiting reagents. Here are the answers:

1. The equation for calculating moles is Mass/Mr.

2. To calculate the number of moles in 1.2 g of sodium hydroxide, you would use the formula: moles = mass/Mr. The molar mass (Mr) of sodium hydroxide (NaOH) is approximately 40 g/mol. So, the calculation would be 1.2 g / 40 g/mol = 0.03 moles.

3. The first thing you should do when calculating reacting masses is to write out the balanced equation.

4. A reactant that is left over at the end of a reaction is called "In excess".

5. In the reaction of Mg with HCl to form MgCl2 and H2, the limiting reagent would be Mg. This is because the balanced chemical equation for this reaction is Mg + 2HCl -> MgCl2 + H2, which means that one mole of Mg reacts with two moles of HCl. So, if you have three moles of Mg, you would need six moles of HCl for the reaction to go to completion. Since you only have five moles of HCl, Mg is the limiting reagent.