Amounts of Substances in Equations Flashcards

All 5 cards in this deck

What do the balancing numbers in a symbol equation tell you?
e.g. $2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$

The ratio of the amounts in moles of reactants and products.
e.g. 2 mol Mg react with 1 mol $\mathrm{O_2}$ to make 2 mol MgO.

What are the steps to calculate the mass of a product from a given mass of reactant?
e.g. mass of MgO from 48 g of Mg in $2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$ ($A_r$ Mg = 24, $M_r$ MgO = 40)

1. moles of reactant = $48 \div 24$ = 2 mol Mg
2. Use the equation ratio (2 : 2) = 2 mol MgO
3. mass = moles $\times M_r$ = $2 \times 40$ = 80 g

What are the steps to calculate the mass of reactant needed to make a given mass of product?
e.g. mass of $\mathrm{CaCO_3}$ needed to make 5.6 g of CaO in $\mathrm{CaCO_3} \rightarrow \mathrm{CaO} + \mathrm{CO_2}$ ($M_r$ CaO = 56, $\mathrm{CaCO_3}$ = 100)

1. moles of product = $5.6 \div 56$ = 0.1 mol CaO
2. Use the equation ratio (1 : 1) = 0.1 mol $\mathrm{CaCO_3}$
3. mass = $0.1 \times 100$ = 10 g

True or false? In $2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$, 2 g of hydrogen reacts with 1 g of oxygen.

False. The balancing numbers give the ratio in moles, not the ratio of masses in grams.

What must you do with a mass given in kilograms before using it in a moles calculation?

Convert it to grams by multiplying by 1000.
e.g. 40.0 kg = 40 000 g.