Chemistry 5070/32 — May/June 2015

Walkthrough

This part is the practical titration itself, so every answer depends on the readings you actually take. P is the sodium hydroxide (the alkali) and Q is the acid $\text{H}_3\text{PO}_3$. A pipette transfers $25.0\text{ cm}^3$ (or $20.0\text{ cm}^3$) of the alkali into a conical flask with a few drops of indicator; the burette contains the acid Q. Run acid from the burette into the flask, swirling, until the indicator just changes colour — the end point. Record the final and initial burette readings; volume of Q used = final − initial. Because one titre can be spoilt, repeat until at least two agree closely. The marker awards three separate batches of marks. (1) Accuracy (up to 4 marks) compares your best titre with the supervisor's value — the closer, the better, with bands at 0.2, 0.3 and 0.4 $\text{cm}^3$. (2) Concordance (up to 3 marks) rewards the ticked values being close to one another — within 0.2 $\text{cm}^3$ for full marks. (3) Average (1 mark) is for a correct average of the ticked values only, error no more than 0.05 $\text{cm}^3$.

Key Takeaways

The skill is clean technique plus honest record-keeping: read the burette to the right precision, repeat to concordance, tick the best results, and average only those.

Common Mistakes

Averaging values you have not ticked; averaging an obvious outlier; ticked results spread by more than 0.2 $\text{cm}^3$; a wrong arithmetic average.

Things to Be Careful About

Two sets of marks depend on your judgement, not the chemistry: concordance uses only the ticked values, and the average must be of exactly those ticked values. Read the burette to the precision it allows and keep the same precision throughout.

Walkthrough

We know the concentration of P ($0.0984\text{ mol / dm}^3$) and the volume pipetted ($25.0\text{ cm}^3$). The fundamental relationship is

The volume must be in $\text{dm}^3$, so $25.0\text{ cm}^3 = \frac{25.0}{1000}\text{ dm}^3$. Then

The mark scheme assumes the $25.0\text{ cm}^3$ pipette; if your pipette is $20.0\text{ cm}^3$, use 20.0 in place of 25.0.

Key Takeaways

Moles = concentration × volume, and the volume must be in $\text{dm}^3$ — a $\text{cm}^3$ figure is always divided by 1000.

Common Mistakes

Using 25.0 directly with the concentration without dividing by 1000 (giving 2.46 mol, a hundred times too big); using the concentration of Q instead of P.

Things to Be Careful About

Keep the unit: the answer is in moles. With the given data you naturally get 5 significant figures, which is fine; quote at least 3.

Walkthrough

Q contains $5.04\text{ g}$ of $\text{H}_3\text{PO}_3$ in every $\text{dm}^3$. The relative formula mass is 82, so one mole of the acid has mass 82 g. The number of moles in $5.04\text{ g}$ is

Because this is per $\text{dm}^3$, the concentration is $0.0615\text{ mol / dm}^3$.

Key Takeaways

Converting a mass concentration ($\text{g / dm}^3$) to a molar concentration ($\text{mol / dm}^3$) is a single division by $M_r$.

Common Mistakes

Dividing the wrong way (82 ÷ 5.04); quoting the answer in $\text{g / dm}^3$ instead of $\text{mol / dm}^3$; quoting more than 3 significant figures.

Things to Be Careful About

Give the unit $\text{mol / dm}^3$ — the question prints the unit but many candidates forget to write it next to the number.

Walkthrough

Now we need the moles of acid in the volume of Q actually delivered in the titration — the average titre from part (a). Using the same relationship, moles = concentration × volume in $\text{dm}^3$:

The average titre is your own value from part (a). Taking the mark scheme's assumed value of $20.2\text{ cm}^3$ as a worked example:

Key Takeaways

Again moles = concentration × volume, and the cm³ titre must be divided by 1000. This part feeds directly into the next one.

Common Mistakes

Plugging the titre in as if it were already in $\text{dm}^3$; using the NaOH concentration instead of the acid concentration; forgetting the /1000.

Things to Be Careful About

Use the average titre from part (a), not a single reading. The final number should have 3 significant figures (0.00124).

Walkthrough

From part (b) we have 0.00246 mol of NaOH in the flask; from part (d) we have the moles of acid delivered by the average titre. The question asks how many moles of NaOH react with 1 mole of $\text{H}_3\text{PO}_3$, so we divide:

The ratio is almost exactly 2, so 2 moles of NaOH react with 1 mole of the acid.

Key Takeaways

A titration gives the mole ratio of the two reactants — moles of one divided by moles of the other, placed in the order the question asks for.

Common Mistakes

Dividing 0.00124 by 0.00246 and getting the ratio inverted; rounding 1.98 to 1 instead of 2.

Things to Be Careful About

The value 1.98 is close to 2 — the small deviation is experimental error, and the sensible reading is exactly 2.

Walkthrough

The ratio from part (e) is 2:1 — two moles of NaOH per mole of acid — so only two of the three hydrogen atoms are acidic in this reaction. Phosphorous acid, $\text{H}_3\text{PO}_3$, is diprotic under these conditions. The coefficient on the left must match your (e) answer, so put 2 in front of NaOH:

Only two H atoms are replaced by sodium, so the salt retains one H — sodium phosphite, $\text{Na}_2\text{HPO}_3$. The remaining H and the extra O from the two NaOH units form 2 water molecules. Check the balance: Na 2/2, H 5/5, O 5/5, P 1/1. The first mark is for whole-number coefficients consistent with (e) on the left-hand side; the second for correct product formulae and the balancing.

Key Takeaways

An experiment can reveal how many acidic hydrogens an acid has. Acid + alkali always gives a salt + water, and here the salt keeps one hydrogen, showing $\text{H}_3\text{PO}_3$ behaves as a diprotic acid in this titration.

Common Mistakes

Writing 3NaOH and $\text{Na}_3\text{PO}_3$ — inconsistent with the 2:1 ratio found; leaving the equation unbalanced; forgetting the water.

Things to Be Careful About

Your coefficients must be consistent with your (e) answer — if the experiment gave a different ratio, your equation would be marked by following on from that value (ecf), so keep both answers in agreement.