ESAT · Chemistry

ESAT chemistry reacting mass questions

Use a balanced equation to find the mass of a product or reactant.

How to spot it

What mass of ... is produced?
... react until one is used up. What mass ...?

Try one

Original example

Magnesium reacts with hydrochloric acid: $\mathrm{Mg}$ ${}+ 2\mathrm{HCl} \rightarrow \mathrm{MgCl_2}$ ${}+ \mathrm{H_2}$ .

What mass of hydrogen gas is produced when $6.0$ g of magnesium reacts completely? ( $A_r$ : H = 1.0, Mg = 24)

The method

Convert every given mass to moles.
Use the equation's ratio; if two amounts are given, find the one that runs out.
Convert the answer back to mass.

The trap

Watch forAssuming both reactants are used up and adding their masses.

Now a harder one

Harder

Hydrogen burns in oxygen: $2\mathrm{H_2}$ ${}+ \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$ .

$4.0$ g of hydrogen and $16$ g of oxygen react until one of them is used up. What mass of water forms? ( $A_r$ : H = 1.0, O = 16)

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