Strong acids and strong bases reaction - calculating pH value

Strong acids and strong bases react and give salts and water as products. Usually strong acids gave less pH values and strong bases have higher pH values. When a base is added to an acid, pH value of acidic solution is increased.


See below tutorial how pH of strong acid is changed when a base is added to the strong acidic solution.

Strong acid and strong bases titration curve

Hydrochloric (HCl), sulfuric acid (H2SO4), nitric acid (HNO3 are some examples to strong acids. Also NaOH, KOH are strong bases.

Learn more about strong acids and strong bases

In this lesson, first we discuss about how to calculate pH of acidic solution of strong acids such as HCl, H2SO4. Then, we study reactions of strong acids and strong bases and calculting pH of thise solutions in different situations.


pH equation

pH = -log10[H3O+(aq)]

Sometimes, we can represent H3O+(aq) as H+(aq)



Example 1

Calculating concentration, pH of hydrochloric acid (HCl)

You are provided 100 cm3 of HCl acid solution. It's concentration is 0.1 mol dm-3.

  1. What is the concentration of H+?
  2. Calculate the amount of moles of H2SO4

Sulfuric acid completely dissociate in the water.

HCl → H+(aq) + Cl-(aq)

Concentration of H+ ions

According to the stoichiometry ratio, concentrations of,
[HCl] = [H3O+(aq)], Therefore,
[HCl] = 0.1 mol dm-3


pH of HCl solution

Now, we know the concentration of H+(aq). Let's substitute that value to the pH equation.

pH = -log10[H3O+(aq)]

pH = -log10[0.1]

pH = 1.0



Example 2

Calculating concentration, pH of sulfuric (H2SO4) acid

You are provided 100 cm3 of H2SO4 acid solution. It's concentration is 0.01 mol dm-3.

  1. What is the concentration of H+?
  2. Calculate the amount of moles of H2SO4

Sulfuric acid completely dissociate in the water.

H2SO4(aq) → 2H+(aq) + SO42-(aq)

Concentration of H+ ions

According to the stoichiometry ratio, concentrations of,
2*[H2SO4(aq)] = [H3O+(aq)], Therefore,
[H3O+(aq)] = 2 * 0.01
[H3O+(aq)] = 0.02 mol dm-3


pH of H2SO4 solution

Now, we know the concentration of H+(aq). Let's substitute that value to the pH equation.

pH = -log10[H3O+(aq)]

pH = -log10[0.02]

pH = 1.7



Example 3

0.1M HCl 50 cm3 solution and 0.05M NaOH 50 cm3 solution is mixed.

  1. Is final solution acidic or basic?
  2. pH of final solution?

Reaction of strong acids and strong bases

An acid and a base(alkali) react and give salt and water as products. pH of acidic solution increases when pouring a base into the acidic solution, and pH of basic solution decreases when pouring an acid into the basic solution. This phenomenon happens due to Neutralization of acid or base.

Download strong acid, base reaction and calculation

HCl is a strong acid. NaOH is a strong base.Hence they react until one reactant finishes. Therfore we have to find what reactant will remain.

  • Calculate amount of moles(n) of each solution.
  • Write the reaction and balance it.
  • Find the ratio that reactants react.
  • Find the reactant which remains in the solution. (HCl or NaOH)
  • Calculate the concentration of remaining reactant.
  • Calculate pH.

Find amount of moles

The relationship of dissolved moles, concentration and volume of solution

concentration equation in solutions

for HCl,
amount of HCl = 0.1 mol dm-3 * 50/1000 dm3
amount of HCl = 0.005 mol

for NaOH,
amount of NaOH = 0.05 mol dm-3 * 50/1000 dm3
amount of NaOH = 0.0025 mol


Equation of NaOH and HCl Reaction and balancing

NaOH and HCl react and give NaCl and H2O as products. NaOH and HCl reacts 1:1 ratio. Therefore same amount of moles of NaOH and HCl will react. Excess reactant(excess moles) will remain in the solution.

equation of NaOH and HCl reaction



Finding remaining reactant in the solution

Construct a simple table which includes total initial moles of each reactant, number of moles reacting(reactants) and producing(products). Then you can calculate remaining reactant. In this example 0.0025 moles of HCl remains in the solution and solution becomes acidic. We don't consider about producing water because reaction occurs in aqueous medium and concentration of water is very high and don't change due to reaction(change is negligible).

finding remining reactant



Finding concentration of remaining reactant in the solution

We know how many moles of HCl remaining in the solution now(0.0025mol) and total volume of the solution(100ml). So, we can calculate the concentration of HCl from concentration equation.

find concentration of HCl solution


Finding pH of HCl solution

HCl is a strong acid. Therefore it dissociates completely into H+ (or H3O+) and Cl- in water. When we write the dissociation equation, we can see the concentration of H3O+ receives from HCl dissociation is equal to the concentration of HCl.

find pH HCl solution



Questions


pH of 0.05 moles of sulphuric acid per 1 dm3

There are 0.05 moles of sulfuric acid 1 dm3 of aqueous solution. To calculate pH, you should know H+ concentration. Sulfuric acid is a strong acid and completely dissociates to H+ ions SO42- ions.

So, if there are 0.05 moles of sulfuric, there are 0.01 moles of H+ ions. Volume of solution is 1 dm3. Then concentration of H+ is 0.1 mol dm-3.

Then pH is 1.






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