In 1884, the Swedish chemist, Arrhenius, proposed a new theory of acids and bases. (i) How did Arrhenius define an acid? (ii) Give one example of a commonly used base and state one use made of it. (b) Define pH. Describe how you could measure the pH of a solution. The concentration of a solution of hydrochloric acid, HCl, is 3.65 grams per litre. (iii) What is the concentration of the solution in moles per litre? (iv) Calculate the pH of the solution.

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In 1884, the Swedish chemist, Arrhenius, proposed a new theory of acids and bases - Leaving Cert Chemistry - Question 7 - 2008

Question 7

In 1884, the Swedish chemist, Arrhenius, proposed a new theory of acids and bases. (i) How did Arrhenius define an acid? (ii) Give one example of a commonly used bas... show full transcript

Worked Solution & Example Answer:In 1884, the Swedish chemist, Arrhenius, proposed a new theory of acids and bases - Leaving Cert Chemistry - Question 7 - 2008

Step 1

How did Arrhenius define an acid?

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Answer

Arrhenius defined an acid as a substance that produces hydrogen ions (H⁺) in solution. This definition emphasizes the role of these ions in creating the acidic properties of the solution. Therefore, an acid can be referred to as a 'proton donor' because it releases protons when dissolved in water.

Step 2

Give one example of a commonly used base and state one use made of it.

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Answer

A commonly used base is sodium hydroxide (NaOH). One common use of sodium hydroxide is for drain cleaning, where it is utilized to break down and dissolve build-up in pipes.

Step 3

Define pH.

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Answer

pH is defined as the negative logarithm of the concentration of hydrogen ions in a solution:

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

This means that pH is a measure of the acidity or alkalinity of a solution, based on the concentration of hydrogen ions present.

Step 4

Describe how you could measure the pH of a solution.

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To measure the pH of a solution, you can use various methods:

pH Meter: This electronic device involves inserting a probe into the solution, which provides a direct reading of the pH.
Universal Indicator Paper: Dip a strip of pH indicator paper into the solution. The color change on the paper can then be compared to a color chart to determine the pH level.

Step 5

What is the concentration of the solution in moles per litre?

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To find the concentration in moles per litre, we use the formula:

$ext{Concentration (M)} = rac{ ext{mass (g)}}{ ext{molar mass (g/mol)} imes ext{volume (L)}}$

For hydrochloric acid (HCl), the molar mass is approximately 36.5 g/mol. Given that we have 3.65 g in 1 L:

$ext{Concentration} = rac{3.65 ext{ g}}{36.5 ext{ g/mol}} = 0.1 ext{ M}$

Step 6

Calculate the pH of the solution.

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Answer

To calculate the pH of a 0.1 M HCl solution, we can use the pH formula:

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

Since HCl is a strong acid, it dissociates completely, meaning the concentration of hydrogen ions [H⁺] is equal to the concentration of the acid:

= -(-1) = 1 $$ Thus, the pH of the solution is 1.

In 1884, the Swedish chemist, Arrhenius, proposed a new theory of acids and bases - Leaving Cert Chemistry - Question 7 - 2008

Worked Solution & Example Answer:In 1884, the Swedish chemist, Arrhenius, proposed a new theory of acids and bases - Leaving Cert Chemistry - Question 7 - 2008

How did Arrhenius define an acid?

Give one example of a commonly used base and state one use made of it.

Define pH.

Describe how you could measure the pH of a solution.

What is the concentration of the solution in moles per litre?

Calculate the pH of the solution.

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