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Acidic, Basic, Neutral Solutions Tutorial

Key Concepts

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Deciding if a Solution is Neutral

A solution is neutral if the concentration of hydrogen ions in solution is the same as the concentration of hydroxide ions in the solution.

neutral solution: [H+] = [OH-]

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Deciding if a Solution is Acidic

A solution is acidic if the concentration of hydrogen ions in the solution is greater than the concentration of hydroxide ions in the solution.

acidic solution: [H+] > [OH-]

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Deciding if a Solution is Basic

A solution is basic if the concentration of hydrogen ions in the solution is less than the concentration of hydroxide ions in the solution:

basic solution: [H+] < [OH-]

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Examples with Worked Solutions

Question 1. A solution is known contain 1.23 × 10-3 mol L-1 hydrogen ions and 1.23 × 10-4 mol L-1 hydroxide ions.
Is the solution acidic, basic or neutral?

  1. Extract the data from the question:
    [H+] = 1.23 × 10-3 mol L-1
    [OH-] = 1.23 × 10-4 mol L-1
  2. Compare [H+] and [OH-]
    [H+] > [OH-]
  3. Decide if the solution is acidic, basic or neutral:
    Solution is acidic because [H+] > [OH-]

Question 2. At 25°C, 10 mL of aqueous sodium hydroxide solution is added to 100 mL of aqueous ethanoic (acetic) acid solution.
The pH of the resulting solution is 3.4.
Is the solution acidic, basic or neutral?

  1. Extract the data from the question:
    volume of NaOH(aq) = 10 mL (not relevant to the question)
    volume of CH3COOH(aq) = 100 mL (not relevant to the question)
    pH of the final solution = 3.4 at 25°C
  2. Calculate the concentration of hydrogen ions in the final solution:
    [H+] = 10-pH = 10-3.4 = 3.98 × 10-4 mol L-1
  3. Calculate the concentration of hydroxide ions in the final solution:
    At 25°C Kw = [H+][OH-] = 1.0 × 10-14
    [OH-] = 1.0 × 10-4 ÷ [H+] = 1.0 × 10-14 ÷ 3.98 × 10-4 = 2.51 × 10-11 mol L-1
  4. Compare [H+] and [OH-] in the final solution:
    [H+] = 3.98 × 10-4 mol L-1
    [OH-] = 2.51 × 10-11 mol L-1
    [H+] > [OH-]
  5. Decide if the solution is acidic, basic or neutral:
    The solution is acidic because [H+] > [OH-]

Question 3. 0.15 g of solid sodium hydroxide is added to 0.025 L of 0.020 mol L-1 HCl(aq).
Is the resulting solution acidic, basic or neutral?

  1. Extract the data from the question:
    mass NaOH = 0.15 g
    volume of HCl(aq) = V(HCl) = 0.025 L
    concentration of HCl(aq) = c(HCl(aq)) = 0.020 mol L-1
  2. Calculate the concentration of hydrogen ions in the solution:
    hydrochloric acid is a strong acid so it fully dissociates in water: HCl → H+(aq) + Cl-(aq)
    [H+(aq)] = [HCl] = 0.020 mol L-1
  3. Calculate moles of NaOH:
    moles = mass ÷ molar mass
    moles(NaOH) = 0.15 g ÷ (22.99 + 16.00 + 1.00) g/mol = 0.15 ÷ 39.99 = 3.75 × 10-3 mol
  4. Calculate theoretical concentration of NaOH when the NaOH is added to the acid, assuming no reaction occurs :
    [NaOH(aq)] = moles ÷ volume (L) = 3.75 × 10-3 mol ÷ 0.025 L = 0.15 mol L-1
  5. Calculate the concentration of OH- due to NaOH once NaOH is added to the acid:
    NaOH is a strong base so it fully dissociates in water: NaOH → Na+(aq) + OH-(aq)
    [OH-(aq)] = [NaOH] = 0.15 mol L-1
  6. Compare [H+(aq)] and [OH-(aq)] in the solution:
    [H+(aq)] = 0.020 mol L-1
    [OH-(aq)] = 0.15 mol L-1
    [H+(aq)] < [OH-(aq)]
  7. Decide if the solution is acidic, basic or neutral:
    The solution is basic because [H+(aq)] < [OH-(aq)]

Question 4. 28.0 mL of 0.012 mol L-1 HCl(aq) is added to 22.0 mL of 0.015 mol L-1 NaOH(aq).
Is the resulting solution acidic, basic or neutral?

  1. Extract the data from the question:
    volume of HCl(aq) = 28.0 mL = 28.0/1000 = 0.0280 L
    concentration of HCl(aq) = 0.012 mol L-1
    volume of NaOH(aq) = 22.0 mL = 22.0/1000 = 0.0220 L
    concentration of NaOH(aq) = 0.015 mol L-1
  2. Calculate the theoretical concentration of hydrogen ions in the final solution resulting from the hydrochloric acid, assuming no reaction occurs:
    moles HCl = concentration (mol/L) × volume (L) = 0.012 mol/L × 0.0280 L = 3.36 × 10-4 mol
    Hydrochloric acid is a strong acid so it fully dissociates in water: HCl → H+(aq) + Cl-(aq)
    moles of H+(aq) = moles HCl = 3.36 × 10-4 mol
    theoretical [H+(aq)] in the final solution = moles H+(aq) ÷ total volume of the solution in litres
    = 3.36 × 10-4 mol ÷ (0.028 L + 0.022 L) = 6.72 × 10-3 mol L-1
  3. Calculate the theoretical concentration of hydroxide ions in the final solution resulting from the sodium hydroxide, assuming no reaction occurs:
    moles NaOH = concentration (mol/L) × volume (L) = 0.015 mol/L × 0.0220 L = 3.30 × 10-4 mol
    Sodium hydroxide is a strong base so it fully dissociates in water: NaOH → Na+(aq) + OH-(aq)
    moles of OH-(aq) = moles NaOH = 3.30 × 10-4 mol
    theoretical [OH-(aq)] in the final solution = moles OH-(aq) ÷ total volume of the solution in litres
    = 3.30 × 10-4 mol ÷ (0.028 L + 0.022 L) = 6.60 × 10-3 mol L-1
  4. Compare [H+(aq)] and [OH-(aq)]
    [H+(aq)] = 6.72 × 10-3 mol L-1
    [OH-(aq)] = 6.60 × 10-3 mol L-1
    [H+(aq)] > [OH-(aq)]
  5. Decide if the solution is acidic, basic or neutral:
    Final solution is acidic because [H+(aq)] > [OH-(aq)]

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1. Since we are using the Arrhenius definition of acids, bases and neutralisation, it is quite acceptable to use H+ (or H+(aq)) to represent the hydrogen ion.

2. If the solution is aqueous, then we can use the terms alkali instead of base and alkaline instead of basic.