smc-3675-45-Standard Solutions

The concentration of hydrochloric acid in a solution was determined by an acid base titration using a standard solution of sodium carbonate.

Explain why sodium carbonate is a suitable compound for preparation of a standard solution. (2 marks)

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A 25.00 mL sample of 0.1050 mol L¯¹ sodium carbonate solution was added to a conical flask and three drops of methyl orange indicator added. The mixture was titrated with the hydrochloric acid and the following readings were recorded.

Using the data from the table, calculate the concentration of the hydrochloric acid. (3 marks)

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Explain the effect on the calculated concentration of hydrochloric acid if phenolphthalein is used as the indicator instead of methyl orange. (2 marks)

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Show Answers Only

a. Suitability of \(\ce{Na2CO3}\):

\(\ce{Na2CO3}\) is a stable compound.
\(\ce{Na2CO3}\) is a pure solid that will not readily absorb water from the atmosphere.
An accurate weight of \(\ce{Na2CO3}\) can therefore be obtained in the experiment’s measurements.

b. 0.2425 mol L^–¹

c. This is a strong acid / weak base titration.

Its equivalence point will occur at a pH less than seven and phenolphthalein changes colour in the pH range 10 – 8.3.
Phenolphthalein indicator would therefore signal the end point before equivalence (i.e. with a lower volume of acid).
The calculated concentration of \(\ce{HCl}\) would be higher than the correct concentration.

Show Worked Solution

a. Suitability of \(\ce{Na2CO3}\):

\(\ce{Na2CO3}\) is a stable compound.
\(\ce{Na2CO3}\) is a pure solid that will not readily absorb water from the atmosphere.
An accurate weight of \(\ce{Na2CO3}\) can therefore be obtained in the experiment’s measurements.

♦ Mean mark (a) 43%.

b. \(\ce{Na2CO3(aq) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + CO2(g)}\)

\[\ce{Average titre = \frac{21.65 + 21.70 + 21.60}{3} = 21.65 mL}\]

\[\ce{n(Na2CO3) = c \times V = 0.1050 \times 0.0250 = 0.002625 mol}\]

\(\ce{n(HCl) = 2 \times n(Na2CO3) = 0.005250 mol}\)

\[\ce{[HCl] = \frac{n}{V} = \frac{0.005250}{0.02165} = 0.2425 mol L^{-1}}\]

c. This is a strong acid / weak base titration.

Its equivalence point will occur at a pH less than seven and phenolphthalein changes colour in the pH range 10 – 8.3.
Phenolphthalein indicator would therefore signal the end point before equivalence (i.e. with a lower volume of acid).
The calculated concentration of \(\ce{HCl}\) would be higher than the correct concentration.

♦♦♦ Mean mark (c) 29%.

CHEMISTRY, M6 2018 HSC 29