The ionic product of water, K_w = 2.93 × 10^{-15} mol² dm^{-6} at 10 °C 0 3 . 1 Which is the correct expression for K_w? Tick (/) one box. A K_w = [H_2O] B K_w = [H^+][OH^-] C K_w = [H^+][H_2O] D K_w = [H^+][OH^-] 0 3 . 2 Calculate the pH of pure water at 10 °C Give your answer to two decimal places. 0 3 . 3 Suggest why this pure water at 10 °C is not alkaline. 0 3 . 4 Calculate the pH of a 0.0131 mol dm^{-3} solution of calcium hydroxide at 10 °C Give your answer to two decimal places. 0 3 . 5 The 0.0131 mol dm^{-3} calcium hydroxide solution at 10 °C was a saturated solution. A student added 0.0131 mol of magnesium hydroxide to 1.00 dm^{-3} of water at 10 °C and stirred the mixture until no more solid dissolved. Predict whether the pH of the magnesium hydroxide solution formed at 10 °C is larger than, smaller than or the same as the pH of the calcium hydroxide solution at 10 °C. Explain your answer.

A-Level AQA Chemistry Fundamentals of Acids & Bases: The ionic product of water, K

The ionic product of water, K_w = 2.93 × 10^{-15} mol² dm^{-6} at 10 °C 0 3 - AQA - A-Level Chemistry - Question 3 - 2017 - Paper 1

Question 3

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The ionic product of water, K_w = 2.93 × 10^{-15} mol² dm^{-6} at 10 °C 0 3 . 1 Which is the correct expression for K_w? Tick (/) one box. A K_w = [H_2O] B K_w = [... show full transcript

Worked Solution & Example Answer:The ionic product of water, K_w = 2.93 × 10^{-15} mol² dm^{-6} at 10 °C 0 3 - AQA - A-Level Chemistry - Question 3 - 2017 - Paper 1

Step 1

Which is the correct expression for K_w?

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Answer

The correct expression for $K_w$ is:

$K_w = [H^+][OH^-]$

This expression indicates that the ionic product of water is the product of the concentrations of hydrogen ions and hydroxide ions.

Step 2

Calculate the pH of pure water at 10 °C

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Answer

The pH of pure water can be calculated using the formula:

$pH = - ext{log} [H^+]$

At 10 °C, the ionic product of water ( $K_w$ ) is:

$K_w = [H^+]^2 = 2.93 imes 10^{-15}$

Thus, we have:

ightarrow ext{sqrt}(2.93 imes 10^{-15}) = 1.71 imes 10^{-7} ext{ mol dm}^{-3} $$ Now substituting into the pH formula gives: $$ pH = - ext{log}(1.71 imes 10^{-7}) = 6.77 $$ Therefore, the pH of pure water at 10 °C is 6.77.

Step 3

Suggest why this pure water at 10 °C is not alkaline.

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Answer

Pure water is neutral, and since the pH is 6.77, it indicates that it is neither acidic nor alkaline. Alkaline solutions typically have a pH greater than 7, while pure water has a balanced concentration of $[H^+]$ and $[OH^-]$ , leading it to be categorized as neutral.

Step 4

Calculate the pH of a 0.0131 mol dm^{-3} solution of calcium hydroxide at 10 °C

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Answer

To find the pH of the calcium hydroxide solution, we first determine the concentration of hydroxide ions ( $[OH^-]$ ):

$[OH^-] = 0.0131 ext{ mol dm}^{-3}.$

Using the relationship of $pOH$ to $[OH^-]$ , we calculate:

$pOH = - ext{log}(0.0131) = 1.83.$

The pH can then be calculated as follows:

$pH = 14 - pOH = 14 - 1.83 = 12.17.$

Therefore, the pH of the calcium hydroxide solution is 12.17.

Step 5

Predict whether the pH of the magnesium hydroxide solution formed at 10 °C is larger than, smaller than or the same as the pH of the calcium hydroxide solution at 10 °C. Explain your answer.

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Answer

The pH of the magnesium hydroxide solution is expected to be smaller than the pH of the calcium hydroxide solution. This is because magnesium hydroxide is less soluble than calcium hydroxide. As such, it will have a lower concentration of hydroxide ions in solution, leading to a lower pH. When dissolved in water, magnesium hydroxide produces fewer $OH^-$ ions compared to calcium hydroxide, which maintains a higher concentration of hydroxide ions in its saturated solution.

The ionic product of water, K_w = 2.93 × 10^{-15} mol² dm^{-6} at 10 °C 0 3 - AQA - A-Level Chemistry - Question 3 - 2017 - Paper 1

Worked Solution & Example Answer:The ionic product of water, K_w = 2.93 × 10^{-15} mol² dm^{-6} at 10 °C 0 3 - AQA - A-Level Chemistry - Question 3 - 2017 - Paper 1

Which is the correct expression for K_w?

Calculate the pH of pure water at 10 °C

Suggest why this pure water at 10 °C is not alkaline.

Calculate the pH of a 0.0131 mol dm^{-3} solution of calcium hydroxide at 10 °C

Predict whether the pH of the magnesium hydroxide solution formed at 10 °C is larger than, smaller than or the same as the pH of the calcium hydroxide solution at 10 °C. Explain your answer.

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