Stoichiometry - The mole and the Avogadro constant (3)
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1.
Question 2
A student titrated 25.0 cm3 of a solution of magnesium hydroxide, Mg(OH)2, with 0.200 mol/dm3 hydrochloric acid (HCl). The reaction is:
Mg(OH)2(aq) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
The endpoint was reached after adding 20.0 cm3 of the HCl solution. Calculate the concentration of the Mg(OH)2 solution in mol/dm3.
Calculation:
- Calculate the moles of HCl used: moles = concentration x volume (dm3). Volume of HCl = 20.0 cm3 = 0.020 dm3. Concentration = 0.200 mol/dm3. Therefore, moles of HCl = 0.200 mol/dm3 x 0.020 dm3 = 0.004 moles.
- Use the stoichiometry of the reaction to find the moles of Mg(OH)2: From the balanced equation, 1 mole of Mg(OH)2 reacts with 2 moles of HCl. Therefore, moles of Mg(OH)2 = 0.004 moles HCl / (2 moles HCl / 1 mole Mg(OH)2) = 0.002 moles Mg(OH)2.
- Calculate the concentration of the Mg(OH)2 solution: Concentration = moles / volume (dm3). Volume of Mg(OH)2 solution = 25.0 cm3 = 0.025 dm3. Therefore, concentration of Mg(OH)2 = 0.002 moles / 0.025 dm3 = 0.08 mol/dm3.
Answer: The concentration of the Mg(OH)2 solution is 0.08 mol/dm3.
2.
Question 2
A compound contains 40% carbon, 6.7% hydrogen and the rest is oxygen. Determine the empirical formula of the compound.
Solution:
- Assume 100g of the compound. This means the mass of carbon is 40g, hydrogen is 6.7g, and oxygen is 100 - 40 - 6.7 = 53.3g.
- Convert the mass of each element to moles:
Moles of C = 40g / 12 g/mol = 3.33 mol
Moles of H = 6.7g / 1 g/mol = 6.7 mol
Moles of O = 53.3g / 16 g/mol = 3.33 mol
- Divide each mole value by the smallest mole value (3.33 mol):
C: 3.33 mol / 3.33 mol = 1
H: 6.7 mol / 3.33 mol = 2
O: 3.33 mol / 3.33 mol = 1
- The empirical formula is CH2O.
Answer: CH2O
3.
A sample of copper(II) carbonate, copper(II) carbonate (CuCO3), has a mass of 12.5 g and contains 65.0% copper by mass. Calculate the mass of copper in the sample.
Given data:
- Mass of copper(II) carbonate (CuCO3) = 12.5 g
- Percentage of copper in CuCO3 = 65.0%
Calculate the mass of copper:
Mass of copper = (Percentage of copper / 100) x Mass of CuCO3
Mass of copper = (65.0 / 100) x 12.5 g = 8.125 g
Answer: The mass of copper in the sample is 8.125 g.