Dilute Strong Acid - Calculate concentration and pH

Strong acids dissociate completely to hydrogen ion and anion. When acid is diluted, concentration decreases and there is clear relationship between pH and number of diluting times. In this tutorial, we are goint to obtain a relationship between diluting factor and pH.


How to dilute an acid?

We can dilute any solution by adding distilled water. In strong acidic solution, when the acidic solution is diluted, H3O+ concentration decreases and pH increases.



Explanation of diluting and effects to the solution

  • No change in amount (mol) of acid
  • But due to change of volume of solution (increase), concentration is changed (reduced).


You should have an understanding about how to find pH of strong acids. If you don't have please refer following tutorial for better undersatnding of this lesson.
Find pH of strong acids



Steps to dilute

  1. Determine the concentration and volume of required diluted acid solution. Then you can know the exact amount of requirement of acid.
  2. Calculate the required volume of strong acid (concentrated) which can give the amount in dilute solution. Use C1V1 = C2V2 relation.
  3. Then separate the required volume from high concentrated solution. Add that solution to water slowly for diluting until required volume of dilute acid is completed.

Strong acids

  • Hydrochloric acid (HCl)
  • Nitric acid (HNO3)
  • Sulfuric acid (H2SO4)
  • Perchloric acid (HClO4)

First, we look some examples which will help to get an idea about concentrations after diluting.


There is a 0.01 mol dm-3 hydrochloric acid 50 cm3 solution. If another 50 cm3 distilled water is added to the hydrochloric acid solution, what is the concentration of acid?


Answer


First calculate the amount of hydrochloric acid.

Amount of HCl = 0.01 mol dm-3 * 50 cm3

Amount of HCl = 0.0005 mol


After adiing 50 cm3 of distilled water total volume of the solution is 100 cm3.

Concentration of final HCl = 0.0005 mol / ( 100 / 1000 ) dm3

Concentration of final HCl = 0.005 mol dm-3


Concentration is reduced by a half when we add same volume (initial HCl solution volume) of distilled water. So concentration is dilluted by two times.


What will happen to H+ concentration after dilution?

H+ concentration also decreases by two times in this example.


What happen to concentration of acid after dilute?

  • Dilute by 2 times, concentration will decrease by two times.
  • dilute by 5 times, concentration will decrease by five times.
  • dilute by 10 times, concentration will decrease by 10 times.

Calculate pH of different strong acid solutions


There are 3 bottles which contain aqueous hydrochloric (HCl) acid solutions. These hydrochloric acids' concentrations in mol dm- are 0.1, 0.01, 0.001. Calculate pH of those solutions.


Hydrochloric acid is a strong acid. Therefore it dissociate completely in the water.

HCl(aq) + H2O(l) → Cl- (aq) + H3O+(aq)



Due to completeness dissociation,

[ HCl(aq)] = [H3O+(aq) ]

Assumption: The H3O+ receive from dissociation of water is negligible when it compares with H3O+ receive from HCl.



strong acid concentrations and pH

Relationship of pH and concentration of weak acid

We can see a pattern of pH values of solutions. pH variation against concentration of strong acids is figured below. you can see diluting 10 times will increase pH value by 1.

strong acid concentrations and pH variation against diluting

pH versus strong acid concentration

When weak acid is diluted by 10 times, concentration of it is decreased by 10 times.

When strong acid is diluted by 10 times, pH value is increased by 1

When strong acid is diluted by 100 times, pH value is increased by 2

When strong acid is diluted by 1000 times, pH value is increased by 3



Example problem:

Calculate concentration and pH of H2SO4 acid

You are provided 10cm3 of 0.1 mol dm-3 solution of H2SO4. You have to do
1. dilute the initial solution by 10 times
2. dilute the initial solution by 100 times
Find the concentration of H2SO4 and pH of every solution.

you can assume that both dissociation of H2SO4 is complete.
Sulfuric acid is strong acid and it dissociates completely in the water and gives two H3O+ (or H+) ions. Therefore concentration of H3O+ is twice as concentration of H2SO4.
[H3O+(aq)] = 2*[H2SO4(aq)]

sulfuric acid dissociation in water

In this example, at initial state,
[H2SO4(aq)] = 0.1 mol dm-3
[H3O +(aq)] = 0.2 mol dm-3

Now we can find the pH of initial solution,
pH = -log10[H3O+(aq)]
pH = -log10[0.2]
pH = 0.699


When initial solution is diluted by 10 times

Concentration of H2SO4(aq) is reduced by 10 times. With that H3O+(aq) concentration is also reduced by 10 times. Then,
[H2SO4(aq)] = 0.01 mol dm-3
[H3O +(aq)] = 0.02 mol dm-3

Now we can find the pH of solution,
pH = -log10[H3O+(aq)]
pH = -log10[0.02]
pH = 1.699


When initial solution is diluted by 100 times

Concentration of H2SO4(aq) is reduced by 100 times. With that H3O+(aq) concentration is also reduced by 100 times. Then,
[H2SO4(aq)] = 0.001 mol dm-3
[H3O +(aq)] = 0.002 mol dm-3

Now we can find the pH of solution,
pH = -log10[H3O+(aq)]
pH = -log10[0.002]
pH = 2.699



Questions


Can a strong acid be dilute?

Both strong acid and weak acid can be dilluted to decrease the concentration. Add distilled water to the stronger acid to dilute.


Is it possible to dilute acid with water

For diluting, distilled water should be used. Normal water contain salt and different constituents.


What is a dilute strong acid

In the laboratories, strong acids have higher concentrations. But in our cases, we do not need that kind of higher concentrations. So we do, take little bit of high concentrated acid and add distilled water to dilute it until our reuired concentration.



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