Titration Calculations: Quiz

Titration Calculations: Quiz

7 questions

1. Question 1EasyTitres: rough 24.10 mL, then 23.45, 23.50 and 23.40 mL. What mean titre should be used?

   A23.45 mLB23.61 mLC24.10 mLD23.50 mL
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   Answer: 23.45 mL

   Leave out the rough titre and average the three concordant ones: (23.45 + 23.50 + 23.40) mL ÷ 3 = 23.45 mL. 23.61 mL includes the rough titre.

2. Question 2EasyHow many moles are in 23.45 mL of 0.1000 mol/L HCl?

   A2.345 molB2.345 × 10⁻³ molC0.2345 molD4.264 × 10⁻³ mol
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   Answer: 2.345 × 10⁻³ mol

   n = c × V = 0.1000 mol/L × 0.02345 L = 2.345 × 10⁻³ mol. The volume must be in litres.

3. Question 3Medium25.00 mL of NaOH needs 23.45 mL of 0.1000 mol/L HCl. What is the NaOH concentration?

   A0.1066 mol/LB0.09380 mol/LC0.04690 mol/LD2.345 mol/L
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   Answer: 0.09380 mol/L

   n(NaOH) = n(HCl) = 2.345 × 10⁻³ mol (1 : 1); c = 2.345 × 10⁻³ mol ÷ 0.02500 L = 0.09380 mol/L.

4. Question 4Medium20.00 mL of H₂SO₄ needs 24.80 mL of 0.1500 mol/L NaOH. What is the acid concentration?

   A0.1860 mol/LB0.09300 mol/LC0.1500 mol/LD0.04650 mol/L
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   Answer: 0.09300 mol/L

   n(NaOH) = 3.720 × 10⁻³ mol; H₂SO₄ : NaOH = 1 : 2, so n(H₂SO₄) = 1.860 × 10⁻³ mol; c = 1.860 × 10⁻³ mol ÷ 0.02000 L = 0.09300 mol/L. 0.1860 forgets the ratio.

5. Question 5MediumA 25.00 mL aliquot taken from a 100.0 mL flask contains 1.760 × 10⁻³ mol of acid. How many moles were in the flask?

   A4.400 × 10⁻⁴ molB1.760 × 10⁻³ molC7.040 × 10⁻³ molD1.760 × 10⁻² mol
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   Answer: 7.040 × 10⁻³ mol

   The flask holds 100.0 ÷ 25.00 = 4 aliquots: 4 × 1.760 × 10⁻³ mol = 7.040 × 10⁻³ mol.

6. Question 6HardIn a back titration, 1.000 × 10⁻² mol HCl is added and 1.240 × 10⁻³ mol is left over. How much HCl reacted with the sample?

   A1.124 × 10⁻² molB8.760 × 10⁻³ molC1.240 × 10⁻³ molD4.380 × 10⁻³ mol
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   Answer: 8.760 × 10⁻³ mol

   Reacted = added − left over = 1.000 × 10⁻² mol − 1.240 × 10⁻³ mol = 8.760 × 10⁻³ mol. 4.380 × 10⁻³ mol is the CaCO₃ it reacts with (1 : 2).

7. Question 7MediumWhy is a back titration used for limestone (CaCO₃)?

   ACaCO₃ is a strong baseBCaCO₃ is insoluble and reacts slowly, so it can't be titrated directlyCThere is no indicator for CaCO₃DIt gives a larger titre
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   Answer: CaCO₃ is insoluble and reacts slowly, so it can't be titrated directly

   An excess of acid is added so the solid reacts completely; the leftover acid is then titrated.