Question

Chemistry calculation help?

Mg + 2 HCl --> MgCl2 + H2

What is the theoretical mass of Mg replacing 1g of H2.

I tried

1 g H2 (1 mol H2 / 2.016 g H2) * (1 mol Mg / 1 mol H2) * (24.305 g Mg / 1 mol Mg) = 12.06 g Mg

However the answer is wrong.....
It told me to use this expression:

(mass of H2 liberated / mass of Mg)

Answer 1

The theoretical mass of `"Mg"` is 12 g.

Explanation:

Method 1. Using moles

The equation for the reaction is

`"Mg + 2HCl → MgCl"_2 + "H"_2`

`" Mass of Mg" = 1 color(red)(cancel(color(black)("g H"_2))) × (1 color(red)(cancel(color(black)("mol H"_2))))/(2.016 color(red)(cancel(color(black)("g H"_2)))) × (1color(red)(cancel(color(black)("mol Mg"))))/(1 color(red)(cancel(color(black)("mol H"_2)))) × "24.30 g Mg"/(1 color(red)(cancel(color(black)("mol Mg")))) = "12 g Mg"`

Method 2. Using molar masses

`M_text(r): color(white)(m) 24.30color(white)(mmmmmmmmm)2.016`
`color(white)(mmmll)"Mg + 2HCl → MgCl"_2 + "H"_2`

`"Mass of Mg" = 1 color(red)(cancel(color(black)("g H"_2))) × "24.30 g Mg"/(2.016 color(red)(cancel(color(black)("g H"_2)))) = "12 g Mg"`

Conclusion:

Both methods are equivalent and equally correct.

If "they" insist you do it "their" way, that's what you must do.

However, there is a saying in English, "There is more than one way to skin a cat."

Your method is correct!