TEST OF PRACTICAL KNOWLEDGE QUESTION
All your burette readings (initial and final), as well as the size of your pipette, must be recorded but on no account of experimental procedure is required. All calculations must be done in your answer book.
A is solution of trioxonitrate (V) acid, B is a solution containing 6.90 g of potassium trioxocarbonate (IV) per dm\(^3\)
(a) Put A into the buret and titrate it against 20.0 cm\(^3\) or 25.0 cm\(^3\) portions of B using methy orange or screened methyl orange as indicater. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume A used. The equation for the reaction is \(\mathrm{K_2CO_{3(aq)} + 2HNO_{3(aq)} \to 2KNO_{3(aq)} + CO_{2(g)} + H_2O_{(l)}}\)
(b) From your results and the information provided calculate;
(i) concenytration of solution B in mol dm\(^{-3}\)
(ii) number of potassium ions in 1.00 dm\(^3\) of B [C = 12.0, O = 16.0, K = 39.0, Avogadro constant = 6.02 x 10\(^{23}\) mol \(^{-1}\)]
(a) Volume of pipette: 25.00 cm3
Indicator used: Methyl orange
End-point colour change: Yellow to orange/pink
| Burette readings |
Rough |
1st |
2nd |
3rd |
| Final reading / cm3 |
16.50 |
16.00 |
15.95 |
26.05 |
| Initial reading / cm3 |
0.00 |
0.00 |
0.00 |
10.00 |
| Volume of A used / cm3 |
16.50 |
16.00 |
15.95 |
16.05 |
Average volume of A used:
\[
\frac{16.00+15.95+16.05}{3}=16.00\ \text{cm}^3
\]
(b)(i) Concentration of B in mol dm-3
Molar mass of K2CO3:
\[
M_r=2(39.0)+12.0+3(16.0)=138.0\ \text{g mol}^{-1}
\]
\[
\text{Concentration of B}=\frac{6.90}{138.0}=0.0500\ \text{mol dm}^{-3}
\]
(b)(ii) Number of potassium ions in 1.00 dm3 of B
In 1.00 dm3 of B:
\[
n(\mathrm{K_2CO_3})=0.0500\ \text{mol}
\]
Each mole of K2CO3 produces 2 moles of K+ ions.
\[
n(\mathrm{K^+})=2\times0.0500=0.100\ \text{mol}
\]
\[
\text{Number of K}^+\text{ ions}=0.100\times6.02\times10^{23}
\]
\[
=6.02\times10^{22}\ \text{potassium ions}
\]
Answers:
Concentration of B = 0.0500 mol dm-3
Number of K+ ions in 1.00 dm3 of B = 6.02 × 1022 ions.
(a) Volume of pipette: 25.00 cm3
Indicator used: Methyl orange
End-point colour change: Yellow to orange/pink
| Burette readings |
Rough |
1st |
2nd |
3rd |
| Final reading / cm3 |
16.50 |
16.00 |
15.95 |
26.05 |
| Initial reading / cm3 |
0.00 |
0.00 |
0.00 |
10.00 |
| Volume of A used / cm3 |
16.50 |
16.00 |
15.95 |
16.05 |
Average volume of A used:
\[
\frac{16.00+15.95+16.05}{3}=16.00\ \text{cm}^3
\]
(b)(i) Concentration of B in mol dm-3
Molar mass of K2CO3:
\[
M_r=2(39.0)+12.0+3(16.0)=138.0\ \text{g mol}^{-1}
\]
\[
\text{Concentration of B}=\frac{6.90}{138.0}=0.0500\ \text{mol dm}^{-3}
\]
(b)(ii) Number of potassium ions in 1.00 dm3 of B
In 1.00 dm3 of B:
\[
n(\mathrm{K_2CO_3})=0.0500\ \text{mol}
\]
Each mole of K2CO3 produces 2 moles of K+ ions.
\[
n(\mathrm{K^+})=2\times0.0500=0.100\ \text{mol}
\]
\[
\text{Number of K}^+\text{ ions}=0.100\times6.02\times10^{23}
\]
\[
=6.02\times10^{22}\ \text{potassium ions}
\]
Answers:
Concentration of B = 0.0500 mol dm-3
Number of K+ ions in 1.00 dm3 of B = 6.02 × 1022 ions.
