# Question 1: 20cm 3 of hydrochloric acid with concentration 0.5 mol/dm 3 is needed to neutralise 25 cm 3 of sodium hydroxide. What is the concentration.

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Question 1: 20cm 3 of hydrochloric acid with concentration 0.5 mol/dm 3 is needed to neutralise 25 cm 3 of sodium hydroxide. What is the concentration of the alkali? Step 1: Write a balanced equation showing the reaction that is happening Step 2: Work out the number of moles of the reactant with the known concentration Step 3: Work out the number of moles for the reactant with the unknown concentration Step 4: Work out the concentration for the reactant whose concentration was unknown NaOH(aq) + HCl(aq)  NaCl(aq) + H 2 O(l) No. of moles of acid: No. of moles = concentration x volume (dm 3 ) No. of moles = 0.5 x 20/1000 No. of moles = 0.01 moles The equation tells us that the acid and alkali react in a 1:1 ratio This means that there are the same no. of moles of acid as there are alkali at neutralisation No. of moles of sodium hydroxide = 0.01 moles No. of moles of sodium hydroxide in 25 cm 3 = 0.01 moles The concentration is the no. of moles in 1000 cm 3 Concentration of sodium hydroxide= 0.01 x 1000/25 = 0.4 mol/dm 3

Question 2: 10cm 3 of sulfuric acid with concentration 1 mol/dm 3 is needed to neutralise 25 cm 3 of potassium hydroxide. What is the concentration of the alkali? Step 1: Write a balanced equation showing the reaction that is happening Step 2: Work out the number of moles of the reactant with the known concentration Step 3: Work out the number of moles for the reactant with the unknown concentration Step 4: Work out the concentration for the reactant whose concentration was unknown 2KOH(aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + 2H 2 O(l) No. of moles of acid: No. of moles = concentration x volume (dm 3 ) No. of moles = 1 x 10/1000 No. of moles = 0.01 moles The equation tells us that the alkali and acid react in a 2:1 ratio This means that there are twice the no. of moles of alkali as there are acid at neutralisation No. of moles of potassium hydroxide = 0.02 moles No. of moles of potassium hydroxide in 25 cm 3 = 0.02 moles The concentration is the no. of moles in 1000 cm 3 Concentration of potassium hydroxide= 0.02 x 1000/25 = 0.8 mol/dm 3

Question 3: 20cm 3 of nitric acid with concentration 0.2 mol/dm 3 is needed to neutralise 15 cm 3 of sodium hydroxide. What is the concentration of the alkali? Step 1: Write a balanced equation showing the reaction that is happening Step 2: Work out the number of moles of the reactant with the known concentration Step 3: Work out the number of moles for the reactant with the unknown concentration Step 4: Work out the concentration for the reactant whose concentration was unknown NaOH(aq) + HNO3(aq)  NaCl(aq) + H 2 O(l) No. of moles of nitric acid: No. of moles = concentration x volume (dm 3 ) No. of moles = 0.2 x 20/1000 No. of moles = 0.004 moles The equation tells us that the acid and alkali react in a 1:1 ratio This means that there are the same no. of moles of acid as there are alkali at neutralisation No. of moles of sodium hydroxide = 0.004 moles No. of moles of sodium hydroxide in 15 cm 3 = 0.004 moles The concentration is the no. of moles in 1000 cm 3 Concentration of sodium hydroxide= 0.004 x 1000/15 = 0.26 mol/dm 3

Question 4: 14cm 3 of hydrochloric acid with unknown concentration is needed to neutralise 25 cm 3 of sodium hydroxide with concentration 0.1 mol/dm 3. What is the concentration of the acid? Step 1: Write a balanced equation showing the reaction that is happening Step 2: Work out the number of moles of the reactant with the known concentration Step 3: Work out the number of moles for the reactant with the unknown concentration Step 4: Work out the concentration for the reactant whose concentration was unknown NaOH(aq) + HCl(aq)  NaCl(aq) + H 2 O(l) No. of moles of alkali: No. of moles = concentration x volume (dm 3 ) No. of moles = 0.1 x 25/1000 No. of moles = 0.0025 moles The equation tells us that the acid and alkali react in a 1:1 ratio This means that there are the same no. of moles of acid as there are alkali at neutralisation No. of moles of hydrochloric acid = 0.0025 moles No. of moles of hydrochloric acid in 14 cm 3 = 0.0025 moles The concentration is the no. of moles in 1000 cm 3 Concentration of hydrochloric acid 0.0025 x 1000/14 = 0.18 mol/dm 3

Question 5: 22cm 3 of sulfuric acid with unknown concentration is needed to neutralise 25 cm 3 of potassium hydroxide with concentration 0.2 mol/dm 3. What is the concentration of the acid? Step 1: Write a balanced equation showing the reaction that is happening Step 2: Work out the number of moles of the reactant with the known concentration Step 3: Work out the number of moles for the reactant with the unknown concentration Step 4: Work out the concentration for the reactant whose concentration was unknown 2KOH(aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + 2H 2 O(l) No. of moles of alkali: No. of moles = concentration x volume (dm 3 ) No. of moles = 0.2 x 25/1000 No. of moles = 0.005 moles The equation tells us that the acid and alkali react in a 1:2 ratio This means that there are twice the no. of moles of alkali as there are acid at neutralisation No. of moles of sulfuric acid = 0.005/2 = 0.0025 moles No. of moles of sulfuric acid in 22 cm 3 = 0.0025 moles The concentration is the no. of moles in 1000 cm 3 Concentration of hydrochloric acid 0.0025 x 1000/22 = 0.11 mol/dm 3

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