Reacting Masses Flashcards

All 3 cards in this deck

What are the steps to calculate the mass of a product from a balanced equation, given the mass of a reactant?

e.g. mass of MgO from 2.4 g Mg in $2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$ ($M_r$: Mg = 24, MgO = 40)

1. Moles of known substance: $2.4 \div 24 = 0.1$
2. Use the equation ratio (2 : 2) to get moles of MgO: $0.1$
3. Mass $=$ moles $\times M_r$: $0.1 \times 40 = 4$ g

True or false? In a reacting mass calculation you can use the ratio of the balancing numbers directly as a ratio of masses.

False. The balancing numbers give a ratio of moles, which must be converted to masses using $M_r$.

How do you convert a mass in kilograms to grams before a mole calculation?

e.g. 2 kg

Multiply by 1000.
$2 \times 1000 = 2000$ g