Photo AI
A student does an experiment to determine the percentage of copper in an alloy - AQA - A-Level Chemistry - Question 6 - 2019 - Paper 1
Question 6
A student does an experiment to determine the percentage of copper in an alloy. The student - reacts 985 mg of the alloy with concentrated nitric acid to form a so... show full transcript
Worked Solution & Example Answer:A student does an experiment to determine the percentage of copper in an alloy - AQA - A-Level Chemistry - Question 6 - 2019 - Paper 1
Step 1
Calculate the percentage of copper by mass in the alloy.
Answer
To find the percentage of copper in the alloy, we start by calculating the amount of copper formed from the given reactions.
-
First, determine the amount of sodium thiosulfate that reacted:
Moles of Na₂S₂O₃ = C imes V = 0.0800 ext{ mol dm}^{-3} imes 0.0900 ext{ dm}^3 = 0.00720 ext{ mol}
-
From the reaction equations, 2 moles of Na₂S₂O₃ react with 1 mole of I₂. So, the moles of I₂ produced is:
Moles of I₂ = 0.00720 ext{ mol Na₂S₂O₃} imes rac{1}{2} = 0.00360 ext{ mol I₂}
-
From the first reaction, we know 1 mole of I₂ comes from 2 moles of Cu²⁺, therefore:
Moles of Cu²⁺ = 2 imes 0.00360 ext{ mol I₂} = 0.00720 ext{ mol Cu²⁺}
-
To find the mass of copper, we use the molar mass of copper which is 63.55 g/mol:
Mass of Cu = 0.00720 ext{ mol} imes 63.55 ext{ g/mol} = 0.457 ext{ g}
-
Next, calculate the percentage of copper in the original alloy sample:
Percentage of Cu = rac{ ext{Mass of Cu}}{ ext{Mass of alloy}} imes 100 oindent = rac{0.457 ext{ g}}{0.985 ext{ g}} imes 100 = 46.4 ext{ %}
Thus, the percentage of copper by mass in the alloy is 46.4%.
Step 2
Suggest two ways that the student could reduce the percentage uncertainty.
Answer
-
Use a larger volume of sodium thiosulfate solution to minimize the relative uncertainty in the measurement.
-
Use a more precise measuring device, such as a burette with finer gradations, to improve the accuracy of volume measurements of the sodium thiosulfate solution.
Step 3
Step 4
Step 5
Step 6
Calculate the volume, in cm³, that 5.00 g of iodine vapor occupies.
Answer
To calculate the volume occupied by 5.00 g of iodine vapor at 185 °C and 100 kPa, we first convert the temperature to Kelvin:
T = 185 + 273.15 = 458.15 ext{ K}
Now, using the ideal gas law, we can calculate the volume:
PV = nRT
Where:
- P = 100 kPa = 100,000 Pa,
- R = 8.31 J K⁻¹ mol⁻¹,
- Molar mass of I₂ = 253.8 g/mol,
- n = rac{5.00 ext{ g}}{253.8 ext{ g/mol}} = 0.0197 ext{ mol}
Substituting into the ideal gas law:
V = rac{nRT}{P} = rac{(0.0197 ext{ mol}) imes (8.31 ext{ J K}^{-1} ext{ mol}^{-1}) imes (458.15 ext{ K})}{100000 ext{ Pa}} = 0.750 imes 10^3 ext{ cm}^3 = 750 ext{ cm}^3
Thus, the volume occupied by iodine vapor is 750 cm³.