pH of Strong Acids and Strong Bases - Calculate pH value

Strong acids show lower pH values while strong bases show higher pH values. Strong acids reacts with strong bases and give salts and water as products. When a strong base is added to a strong acid, pH value of acidic solution is increased. Also when a strong acid is added to a strong base, pH value of strong base solution is decreased.


Content

  • Examples for strong acids
  • Examples for strong bases
  • pH equation
  • pOH equation
  • pH + pOH = pKw
    • pH + pOH = pKw for a neutralized solution at 250C<
  • Calculate pH of strong acids solutions
    • Calculate pH of Hydrochloric acid solution
    • Calculate pH of Sulfuric acid solution
  • Calculate pH of strong bases solutions
    • Calculate pH of Sodium hydroxide solution
    • Calculate pH of Barium hydroxide solution
  • Reaction of strong acids and strong bases



Refer following tutorial to how pH value of the acidic solution is changed when a strong base is added to the strong acid solution

Strong acid and strong bases titration curve

Examples for strong acids

Hydrochloric (HCl), sulfuric acid (H2SO4), nitric acid (HNO3 are some examples to strong acids. These acids completely hydrolysis to hydronium ions and respective anions. Therefore, Hydronium ion concentration is higher than a Hydronium ion concentration of a weak acid.



Examples for strong bases

Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Barium hydroxide several are strong bases.



Learn more about strong acids and strong bases

In this lesson, first we discuss about how to calculate pH of acidic solutions of strong acids such as HCl, H2SO4. Then, we can find pH values of bases. Finally, we study reactions of strong acids and strong bases and calculating pH of these solutions in different situations.




pH equation

pH = -log10[H3O+(aq)]

Hydronium ion (H3O+) concentration should be substituted in mol dm-3 to the pH equation. Sometimes, we can represent H3O+(aq) as H+(aq) and revised equation can be presented as below.

pH = -log10[H+(aq)]



pOH equation

pOH = -log10[OH-(aq)]

Hydroxyl ion (OH-) concentration should be substituted in mol dm-3 to the pOH equation.



pH + pOH = pKw equation

Summation of pH and pOH value is a constant in a constant temperature. In 250C, pKa value is 14.


pH + pOH = pKw for a neutralized solution at 250C

  • In a neutralized aqueous solution, H3O+ concentration and OH- concentration is equal to the 1 * 10-7 mol dm-3.
  • Substitute those values in pH and pOH equation. You will get both pH and pOH values as 7. Therefore summation of pH and pOH values will be fourteen.



Calculate pH of strong acids solutions


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

Hydrochloric acid completely dissociates to hydronium ions and chloride ions 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




Calculate pH of strong bases solutions

Strong bases completely dissociates to ions in the water. Therefore, there is higher hydroxyl ion concentration in the aqueous solution. Aqueous strong base solutions show higher pH values.


Steps of calculating pH value of strong base

  1. Calculating pOH of strong base
  2. Calculating pH by using pH + pOH = pKa


Example 3: Find pH of 0.1 mol dm-3 Sodium hydroxide solution at 250C

pH value of a strong base is found in two steps as mentioned above.


Find pOH

  • Sodium hydroxide completely dissociates to Sodium cations and hydroxyl anions as following equation.
  • NaOH(aq) → Na+(aq) + OH-(aq)
  • According to the stoichiometric balanced equation of dissociation, hydroxyl ion concentration is same as the to the initial concentration of base.
  • Therefore, OH- concentration is 0.1 mol dm-3.
  • Substitute OH- concentration to the pOH equation
    • pOH = -log10[OH-(aq)]
    • pOH = -log10(0.1)
    • pOH = 1

Apply pH + pOH = pKw equation

Because, pOH value is found in the last step and pKw value is 14 at 250C,

  • pH + pOH = pKw
  • pH + 1 = 14
  • pH = 13

Therefore, 0.1 mol dm-3 NaOH solution is a strong basic solution.




Calculate pH after mixing a strong base and a strong base

here, we try to calculate pH value when a strong acid is mixed with a strong base or vice versa.



Example 4: 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. Therefore, we have to find what reactant will remain.


Steps of calculation

  • 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 can be given like this.

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 balanced equation

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 (0.0025 mol) 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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