Calculations from Titrations

Quantitative Chemistry

What is the mean titre?

The average of the concordant titres from a titration.

Which titres should be used when calculating a mean titre?

Only concordant titres should be used.

What are concordant titres?

Titres that are within 0.20 cm³ of each other.

Should a rough titre normally be included when calculating the mean titre?

No, because it is often less accurate and may overshoot the end point.

How do you calculate a titre from burette readings?

Titre = final burette reading − initial burette reading.

How do you calculate the mean titre?

Add the concordant titres together and divide by the number of concordant titres.

If concordant titres are 22.60, 22.70 and 22.65 cm³, what is the mean titre?

(22.60 + 22.70 + 22.65) ÷ 3 = 22.65 cm³.

Why is the first titration often not concordant?

It is commonly used as a rough titration to estimate the end point and may be overshot.

Why is a rough titration useful?

It shows approximately where the end point occurs, allowing later titrations to be carried out more accurately.

What equation is used to calculate the amount in moles from concentration and volume?

n = cV.

What does n represent in n = cV?

Amount of substance in moles (mol).

What does c represent in n = cV?

Concentration in mol dm⁻³.

What does V represent in n = cV?

Volume in dm³.

How do you convert a volume from cm³ to dm³?

Divide the volume in cm³ by 1000.

How do you convert a volume from dm³ to cm³?

Multiply the volume in dm³ by 1000.

What is 25.0 cm³ in dm³?

0.0250 dm³.

What is 22.65 cm³ in dm³?

0.02265 dm³.

What equation is used to calculate concentration?

c = n ÷ V.

What are the units of concentration in titration calculations?

mol dm⁻³.

What are the main steps in a titration concentration calculation?

Calculate the moles of the known substance, use the balanced equation to find the moles of the unknown substance, then calculate its concentration using c = n/V.

What must you write before using the reacting ratio in a titration calculation?

A balanced chemical equation.

Why is the balanced equation important in titration calculations?

Its coefficients give the mole ratio between the reacting substances.

What does a 1:1 reacting ratio mean?

One mole of the first substance reacts with one mole of the second substance.

What does a 1:2 reacting ratio mean?

One mole of the first substance reacts with two moles of the second substance.

If the reacting ratio is 1:1 and there are 0.00223 mol of acid, how many moles of base react?

0.00223 mol.

What is the equation for the reaction between sodium hydroxide and sulfamic acid?

NaOH + NH₂SO₃H → NH₂SO₃Na + H₂O.

What is the mole ratio of NaOH to NH₂SO₃H?

1:1.

If 22.65 cm³ of 0.0985 mol dm⁻³ sulfamic acid is used, how many moles of sulfamic acid are present?

n = 0.0985 × 0.02265 = 0.00223 mol.

If 0.00223 mol NaOH is contained in 25.0 cm³, what is its concentration?

c = 0.00223 ÷ 0.0250 = 0.0892 mol dm⁻³.

What is the equation for the reaction between sodium carbonate and nitric acid?

Na₂CO₃ + 2HNO₃ → 2NaNO₃ + H₂O + CO₂.

What is the mole ratio of Na₂CO₃ to HNO₃?

1:2.

If 25.0 cm³ of 0.108 mol dm⁻³ Na₂CO₃ is used, how many moles of Na₂CO₃ are present?

n = 0.108 × 0.0250 = 0.00270 mol.

If there are 0.00270 mol Na₂CO₃, how many moles of HNO₃ react with it?

0.00270 × 2 = 0.00540 mol HNO₃.

If 0.00540 mol HNO₃ is contained in 27.25 cm³, what is its concentration?

c = 0.00540 ÷ 0.02725 = 0.198 mol dm⁻³.

What is the equation for the reaction between sodium hydroxide and hydrochloric acid?

NaOH + HCl → NaCl + H₂O.

What is the reacting ratio of NaOH to HCl?

1:1.

What shortcut equation can be used for titration calculations?

(V₁ × M₁) / n₁ = (V₂ × M₂) / n₂.

What does V represent in the titration shortcut equation?

Volume of the solution.

What does M represent in the titration shortcut equation?

Molar concentration of the solution.

What does n represent in the titration shortcut equation?

The coefficient of the substance in the balanced chemical equation.

In the shortcut equation, what do subscripts 1 and 2 represent?

The first and second substances in the balanced chemical equation.

When can cm³ be used directly in the titration shortcut equation?

When both volumes are in the same units, because the volume conversion factors cancel.

If 25.0 cm³ of 0.0892 mol dm⁻³ NaOH reacts with 22.68 cm³ HCl in a 1:1 ratio, what is the HCl concentration?

(25.0 × 0.0892) ÷ 22.68 = 0.0983 mol dm⁻³.

What is the safest general method for titration calculations?

Use moles: calculate n = cV for the known solution, apply the mole ratio, then use c = n/V for the unknown solution.

Why is the moles method useful for titration calculations?

It works for both simple and more complicated reacting ratios.

What common mistake can occur when using reacting ratios?

Using the ratio the wrong way around.

How can you avoid using the mole ratio the wrong way around?

Write the balanced equation and identify which substance corresponds to each coefficient before calculating.

What equation represents the reaction between potassium hydroxide and sulfuric acid?

2KOH + H₂SO₄ → K₂SO₄ + 2H₂O.

What is the mole ratio of KOH to H₂SO₄?

2:1.

If you know the moles of H₂SO₄ in the reaction 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O, how do you find the moles of KOH?

Multiply the moles of H₂SO₄ by 2.

What should you check before giving your final answer in a titration calculation?

Check the balanced equation, mole ratio, volume conversion and units.

What units should the final concentration normally have?

mol dm⁻³.

What is a common error when using n = cV with a volume given in cm³?

Forgetting to divide the volume by 1000 to convert it to dm³.