Question

How many moles of magnesium (if any) remain when 5 g of Mg is burned in 2.5g of pure O2 to make magnesium oxide? Answer is 0.0498 mol Please show your work

Answer 1

Molar mass of Mg = 24.31 g/mol

mass of Mg = 5.0 g

we have below equation to be used:

number of mol of Mg,

n = mass of Mg/molar mass of Mg

=(5.0 g)/(24.31 g/mol)

= 0.2057 mol

Molar mass of O2 = 32 g/mol

mass of O2 = 2.5 g

we have below equation to be used:

number of mol of O2,

n = mass of O2/molar mass of O2

=(2.5 g)/(32 g/mol)

= 7.812*10^-2 mol

we have the Balanced chemical equation as:

2 Mg + O2 ---> 2 MgO +

2 mol of Mg reacts with 1 mol of O2

for 0.2057 mol of Mg, 0.1028 mol of O2 is required

But we have 0.0781 mol of O2

so, O2 is limiting reagent

we will use O2 in further calculation

From balanced chemical reaction, we see that

when 1 mol of O2 reacts, 2 mol of Mg is formed

mol of Mg reacted = (2/1)* moles of O2

= (2/1)*0.0781

= 0.1562 mol

mol of Mg remaining = mol initially present - mol reacted

mol of Mg remaining = 0.2057 - 0.1562

mol of Mg remaining = 0.0494 mol

Answer: 0.0494 mol