Calculating Mass of Substances Flashcards

All 25 cards in this deck

What is the unit used to measure chemical amounts, and what is its symbol?

The mole, symbol $\mathrm{mol}$.

What is the mass of one mole of a substance in grams?

Numerically equal to its relative formula mass ($M_r$).
e.g. $M_r$ of $\mathrm{CO_2}$ = 44, so one mole has a mass of 44 g.

What is the Avogadro constant?

$6.02 \times 10^{23}$ per mole — the number of atoms, molecules or ions in one mole of a substance.

What are the steps to calculate the number of moles in a given mass of a substance?
e.g. 88 g of $\mathrm{CO_2}$ ($M_r$ = 44)

1. Find the $M_r$: 44
2. moles = mass ÷ $M_r$ = $88 \div 44$ = 2 mol

What are the steps to calculate the number of particles in a given mass of a substance?
e.g. molecules in 88 g of $\mathrm{CO_2}$ ($M_r$ = 44)

1. moles = mass ÷ $M_r$ = $88 \div 44$ = 2 mol
2. particles = moles $\times 6.02 \times 10^{23}$ = $1.204 \times 10^{24}$ molecules

True or false? One mole of carbon (C) contains the same number of atoms as the number of molecules in one mole of carbon dioxide ($\mathrm{CO_2}$).

True. One mole of any substance contains the same number of the stated particles as one mole of any other substance.

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.

What are the steps to work out the balancing numbers of an equation from the masses of reactants and products?
e.g. 4.8 g Mg + 3.2 g $\mathrm{O_2}$ → 8.0 g MgO

1. Convert each mass to moles: $4.8/24 = 0.2$, $3.2/32 = 0.1$, $8.0/40 = 0.2$
2. Divide by the smallest to get a whole number ratio: 2 : 1 : 2
3. Write the equation: $2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$

How do you turn a set of mole values into simple whole number balancing numbers?
e.g. 0.25 mol : 0.75 mol

Divide them all by the smallest value (then multiply up if needed).
e.g. $0.25 : 0.75$ → 1 : 3

True or false? The balancing numbers of an equation can be found from the ratio of the masses of the reactants and products in grams.

False. The masses must first be converted to amounts in moles, and that ratio simplified.

What is the limiting reactant?

The reactant that is completely used up, which limits the amount of product formed.

What does it mean to say a reactant is in excess?

There is more of it than is needed to react with all of the other reactant, so some is left over.

What are the steps to identify the limiting reactant?
e.g. 0.2 mol Mg with 0.05 mol $\mathrm{O_2}$ in $2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$

1. Find the moles of each reactant: 0.2 mol Mg, 0.05 mol $\mathrm{O_2}$
2. Divide each by its balancing number: $0.2/2 = 0.1$ and $0.05/1 = 0.05$
3. The smallest value is the limiting reactant: $\mathrm{O_2}$

Which reactant's amount do you use to calculate the maximum mass of product?

The limiting reactant, because it is used up first and limits the amount of product.

True or false? The reactant with the smaller mass is always the limiting reactant.

False. You must compare the amounts in moles against the ratio in the balanced equation.

What is the formula for the concentration of a solution in g/dm$^3$?

concentration = mass of solute (g) ÷ volume of solution (dm$^3$)

How many cm$^3$ are there in 1 dm$^3$?

1000 cm$^3$.

What are the steps to calculate a concentration in g/dm$^3$ when the volume is given in cm$^3$?
e.g. 5 g of solute in 250 cm$^3$ of solution

1. Convert the volume to dm$^3$: $250 \div 1000 = 0.25$ dm$^3$
2. concentration = mass ÷ volume = $5 \div 0.25$ = 20 g/dm$^3$

What are the steps to calculate the mass of solute in a given volume of solution of known concentration?
e.g. 500 cm$^3$ of a 20 g/dm$^3$ solution

1. Convert the volume to dm$^3$: $500 \div 1000 = 0.5$ dm$^3$
2. mass = concentration $\times$ volume = $20 \times 0.5$ = 10 g

True or false? Adding more water to a solution, keeping the mass of solute the same, decreases its concentration.

True. A larger volume of solution for the same mass of solute means a lower concentration.

How does concentration change if the mass of solute dissolved is increased but the volume of solution stays the same?

The concentration increases, in proportion to the mass of solute.