Reacting masses.

Step 1 of 5

Relative molecular mass

The coefficients in an equation count particles: molecules, atoms or units of a salt. In the lab you weigh grams. To go from one to the other you need to know how much each particle weighs.

A substance’s relative molecular mass is the sum of the relative atomic masses of every atom in its formula. It is also the molar mass, in grams per mole: one mole of the substance weighs that number of grams.

Step 2 of 5

Gram for gram

Because every unit of a compound is built from the same atoms, the ratio by mass between its elements is fixed too. That is the law of definite proportions: in every sample of $\mathrm{MgCl_2}$⁠, from any source, there are the same grams of chlorine for each gram of magnesium.

The readings beside the Scene show that ratio on their last line. Use it.

Step 3 of 5

The product’s mass

Conservation of mass works in grams too: every gram of magnesium and every gram of chlorine that reacted is now in the salt.

Step 4 of 5

Through moles

The ratio in the readings works for one compound. The general way, for any reaction, goes through moles, in three steps:

1. Grams of the known substance, over its molar mass, give moles: $n = \dfrac{m}{M}$⁠.
2. The coefficients in the equation say how many moles of the substance you want match them.
3. Moles of the substance you want, times its molar mass, give grams: $m = n \times M$⁠.

Compound is now fixed at Water: $\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$⁠.

Step 5 of 5

The other way round

The same three steps work from the product back to the reactants: start from the mass you want to make, and work back to how much you need.