Quantitative Analysis Flashcards

All 39 cards in this deck

What does a concentration of $1\ \mathrm{mol\ dm^{-3}}$ mean?

1 mole of solute is dissolved in every $1\ \mathrm{dm^3}$ ($1000\ \mathrm{cm^3}$) of solution.

What are the steps to calculate the concentration of a solution in $\mathrm{mol\ dm^{-3}}$ from the mass of solute?

e.g. $4.0\ \mathrm{g}$ of NaOH ($M_r = 40$) dissolved to make $500\ \mathrm{cm^3}$ of solution

1. Moles = mass $\div M_r$ = $4.0 \div 40 = 0.10$ mol
2. Volume in $\mathrm{dm^3}$ = $500 \div 1000 = 0.50$
3. Concentration = moles $\div$ volume = $0.10 \div 0.50 = 0.20\ \mathrm{mol\ dm^{-3}}$

What are the steps to convert a concentration in $\mathrm{g\ dm^{-3}}$ into $\mathrm{mol\ dm^{-3}}$?

e.g. $9.80\ \mathrm{g\ dm^{-3}}$ of KOH ($M_r = 56$)

1. Divide the concentration in $\mathrm{g\ dm^{-3}}$ by the $M_r$
2. $9.80 \div 56 = 0.175\ \mathrm{mol\ dm^{-3}}$

What are the steps to convert a concentration in $\mathrm{mol\ dm^{-3}}$ into $\mathrm{g\ dm^{-3}}$?

e.g. $0.175\ \mathrm{mol\ dm^{-3}}$ KOH ($M_r = 56$)

1. Multiply the concentration in $\mathrm{mol\ dm^{-3}}$ by the $M_r$
2. $0.175 \times 56 = 9.80\ \mathrm{g\ dm^{-3}}$

True or false? To change a volume in $\mathrm{cm^3}$ into $\mathrm{dm^3}$ you multiply by 1000.

False. You divide by 1000 ($1\ \mathrm{dm^3} = 1000\ \mathrm{cm^3}$).

What are the steps to carry out an accurate acid–alkali titration?

e.g. finding the volume of dilute hydrochloric acid needed to neutralise $25.0\ \mathrm{cm^3}$ of sodium hydroxide solution

1. Use a pipette to transfer $25.0\ \mathrm{cm^3}$ of alkali into a conical flask and add a few drops of indicator
2. Add acid from a burette, swirling the flask over a white tile, dropwise near the end point, until the indicator just changes colour
3. Repeat until titres are concordant (within $0.20\ \mathrm{cm^3}$) and calculate the mean titre

In a titration, what colour change does phenolphthalein show when acid is added to an alkali?

Pink $\rightarrow$ colourless.

In a titration, what colour change does methyl orange show when acid is added to an alkali?

Yellow $\rightarrow$ orange (red in excess acid).

What makes titration titres concordant, and what are they used for?

Titres within $0.20\ \mathrm{cm^3}$ of each other; they are used to calculate the mean titre.

True or false? Water left in the conical flask after rinsing makes the titre too low.

False. The amount of alkali in the flask is unchanged, so there is no effect on the titre.

How should a solution of accurately known concentration be prepared for a titration?

Dissolve the solid in distilled (deionised) water and make the solution up to the mark in a volumetric flask, e.g. $250\ \mathrm{cm^3}$ in total.

What is the formula linking moles, concentration and volume of a solution?

e.g. $25.0\ \mathrm{cm^3}$ of $0.100\ \mathrm{mol\ dm^{-3}}$ hydrochloric acid

moles = concentration $\times$ volume in $\mathrm{dm^3}$ = $\dfrac{\text{conc} \times \text{vol in cm}^3}{1000}$
$= \dfrac{0.100 \times 25.0}{1000} = 0.00250$ mol

What are the steps to calculate an unknown concentration from titration results?

e.g. $25.0\ \mathrm{cm^3}$ of NaOH needs $20.0\ \mathrm{cm^3}$ of $0.100\ \mathrm{mol\ dm^{-3}}$ HCl (NaOH + HCl $\rightarrow$ NaCl + H$_2$O)

1. Moles of acid = $\dfrac{0.100 \times 20.0}{1000} = 0.00200$ mol
2. Use the ratio in the equation: 1 : 1, so moles of NaOH = $0.00200$ mol
3. Concentration = $\dfrac{0.00200 \times 1000}{25.0} = 0.0800\ \mathrm{mol\ dm^{-3}}$

What are the steps to calculate the volume of a solution needed for complete reaction?

e.g. what volume of $0.200\ \mathrm{mol\ dm^{-3}}$ HCl reacts with $25.0\ \mathrm{cm^3}$ of $0.100\ \mathrm{mol\ dm^{-3}}$ NaOH

1. Moles of NaOH = $\dfrac{0.100 \times 25.0}{1000} = 0.00250$ mol
2. Use the ratio in the equation: 1 : 1, so moles of HCl = $0.00250$ mol
3. Volume = $\dfrac{0.00250 \times 1000}{0.200} = 12.5\ \mathrm{cm^3}$

True or false? In the titration $\mathrm{H_2SO_4 + 2KOH \rightarrow K_2SO_4 + 2H_2O}$, the moles of KOH equal the moles of $\mathrm{H_2SO_4}$.

False. The ratio is 2 : 1, so moles of KOH = $2 \times$ moles of $\mathrm{H_2SO_4}$.

What is the formula for percentage yield?

$\text{percentage yield} = \dfrac{\text{actual yield}}{\text{theoretical yield}} \times 100$

What is meant by the theoretical yield of a reaction?

The maximum mass of product that could be made from the given amounts of reactants, calculated from the balanced equation.

State three causes of the actual yield of a reaction being less than the theoretical yield.

Incomplete reaction; practical losses during the experiment; competing, unwanted side reactions.

What are the steps to calculate percentage yield from a balanced equation?

e.g. the equation predicts $0.20$ mol of a product of $M_r = 80$, and $12\ \mathrm{g}$ is actually made

1. Use the equation ratio to find moles of product: $0.20$ mol
2. Theoretical yield = moles $\times M_r$ = $0.20 \times 80 = 16\ \mathrm{g}$
3. Percentage yield = $\dfrac{12}{16} \times 100 = 75\%$

True or false? A percentage yield greater than 100% shows the reaction was very efficient.

False. The actual yield cannot exceed the theoretical yield — a value over 100% means the product is impure or still wet.

What does the atom economy of a reaction tell you?

The percentage of the mass of the reactants that ends up as the desired product.

What are the steps to calculate the atom economy of a reaction?

e.g. $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$, desired product CaO ($M_r$: CaCO$_3$ = 100, CaO = 56)

1. $M_r$ of desired product = $56$
2. Sum of $M_r$ of all reactants = $100$
3. Atom economy = $\dfrac{56}{100} \times 100 = 56\%$

True or false? A reaction whose only product is the desired product has an atom economy of 100%.

True. All the atoms of the reactants end up in the desired product.

True or false? A reaction with a high atom economy must also have a high percentage yield.

False. Atom economy depends on the equation, while yield depends on how much product is actually obtained.

What is the advantage of a reaction pathway with a high atom economy?

Less waste is produced, so raw materials are used more efficiently.

Which five types of data are used to justify the choice of a reaction pathway for making a product?

Atom economy, percentage yield, rate of reaction, position of equilibrium and usefulness of the by-products.

True or false? The reaction pathway with the highest percentage yield is always the one chosen in industry.

False. Atom economy, rate, equilibrium position and useful by-products are also considered.

Why might a pathway with a lower atom economy still be chosen?

Because its by-products are useful and can be sold or used elsewhere, so little is wasted.

When comparing pathways, what does a position of equilibrium far to the right indicate?

More of the reactants are converted to products, giving a higher yield.

When choosing a reaction pathway, why is a fast rate of reaction an advantage?

More product is made in a given time, lowering production costs.

What is the molar volume of a gas at room temperature and pressure?

$24\ \mathrm{dm^3}$ ($24\,000\ \mathrm{cm^3}$) — the volume occupied by one mole of molecules of any gas at rtp.

What is the formula linking the volume of a gas at rtp to the number of moles?

e.g. $0.50$ mol of carbon dioxide

volume in $\mathrm{dm^3}$ = moles $\times 24$
$= 0.50 \times 24 = 12\ \mathrm{dm^3}$

What are the steps to find the number of moles in a measured volume of gas at rtp?

e.g. $480\ \mathrm{cm^3}$ of hydrogen

1. Use moles = volume $\div$ molar volume, in matching units
2. $480 \div 24\,000 = 0.020$ mol

What are the steps to find the volume of gas produced from a mass of solid?

e.g. $10\ \mathrm{g}$ of CaCO$_3$ ($M_r = 100$) decomposing: $\mathrm{CaCO_3 \rightarrow CaO + CO_2}$

1. Moles of solid = mass $\div M_r$ = $10 \div 100 = 0.10$ mol
2. Use the equation ratio (1 : 1): moles of CO$_2$ = $0.10$ mol
3. Volume = moles $\times 24 = 0.10 \times 24 = 2.4\ \mathrm{dm^3}$

True or false? At rtp, one mole of carbon dioxide occupies a larger volume than one mole of hydrogen.

False. One mole of any gas occupies the same volume, $24\ \mathrm{dm^3}$, at rtp.

State Avogadro's law.

Equal volumes of different gases at the same temperature and pressure contain equal numbers of molecules.

In a reaction between gases at the same temperature and pressure, what does the ratio of their volumes equal?

The mole ratio of the gases in the balanced equation.

What are the steps to use Avogadro's law to find the volume of a gaseous product?

e.g. $30\ \mathrm{cm^3}$ of hydrogen reacting completely in $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$

1. Read the mole ratio from the equation: H$_2$ : NH$_3$ = 3 : 2
2. Gas volumes are in the same ratio as the moles
3. Volume of NH$_3$ = $30 \times \frac{2}{3} = 20\ \mathrm{cm^3}$

True or false? Avogadro's law can be used to compare the volumes of solid reactants as well as gases.

False. It applies only to gases at the same temperature and pressure.