Acid

When acids dissolve in water, they produce protons (H⁺ ions). These protons make the solution acidic.

Example)

HCl(aq) → H⁺(aq) + Cl^-(aq)

H₂SO₄(aq) → 2H⁺(aq) + SO₄^2-(aq)

Alkali

When alkalis dissolve in water, they produce hydroxide ions (OH^-). These hydroxide ions make the solution alkaline.

Example)

NaOH(aq) → Na⁺(aq) + OH^-(aq)

Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH^-(aq)

Tests for Acids and Alkalis

Litmus

- Litmus indicator solution turns red in acidic solutions, and turns blue in alkaline solutions.

- Litmus papers are also used.

Phenolphthalein and Methyl Orange

Universal Indicator and pH scale

pH scale (0 to 14) shows the strength of acidic or alkaline solutions and can be measured by a universal indicator.

• acidic solution: pH is smaller than 7. The smaller the pH, the more acidic the solution.
• neutral solution: pH is 7.
• alkaline solution: pH is larger than 7. The bigger the pH, the more alkaline the solution.

Neutralization Reaction

Neutralization reaction between an acid and an alkali produces a salt and water.

Acid + Base → Salt + Water

This is because hydrogen ion from the acid and hydroxide ion from alkali react to form water.

H⁺(aq) + OH^-(aq) → H₂O (l)

Example of neutralization reaction:

H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

HCl(aq) + NaOH (aq) → NaCl(aq) + H₂O(l)

Acid-Alkali Titration

Titration is a method where a solution of known concentration is used to determine the concentration of another solution. The solution is added slowly to another solution until neutralization happens.

Example

A student carries out a titration to find the concentration of some dilute sulfuric acid. She is given

• a supply of phosphoric acid (H₃PO₄)
• sodium hydroxide (NaOH) solution of concentration 0.180 mol/dm³ 
• apparatus suitable for carrying out a titration
• phenolphthalein indicator

She uses this method to do the titration.

1. Using a pipette, add 25.0 cm³ of the sodium hydroxide solution to a conical flask.
2. Add drops of phenolphthalein indicator to the conical flask.
3. Fill a burette with the phosphoric acid.
4. Slowly add phosphric acid to the conical flask until the phenolphthalein indicator just changes color from pink to colorless. At this point, the solution has just become neutral.

After several repeats, the student calculated the average volume of phosphoric acid added to be 28.30 cm³.

(a) Find the balanced chemical equation for the neutralization reaction.

3NaOH + H₃PO₄ → Na₃PO₄ + 3H₂O

(b) Calculate the amount, in moles, of NaOH in 25.0 cm³ of the sodium hydroxide solution.

0.180 x 25.0 / 1000 = 0.00450 moles

(c) Calculate the amount, in moles, of H₃PO₄ in the phosphoric acid solution.

0.00450 / 3 = 0.00150 moles

(d) Calculate the concentration, in mol/dm³ , of the phosphoric acid.

0.00150 x 1000 / 28.30 = 0.0530 mol/dm³

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