# Acid-base reactions  (Page 4/5)

 Page 4 / 5
When you are busy with these calculations, you will need to remember the following:

1dm ${}^{3}$ = 1 litre = 1000ml = 1000cm ${}^{3}$ , therefore dividing cm ${}^{3}$ by 1000 will give you an answer in dm ${}^{3}$ .

Some other terms and equations which will be useful to remember are shown below:

• Molarity is a term used to describe the concentration of a solution, and is measured in mol.dm ${}^{-3}$ . The symbol for molarity is M. Refer to chapter [link] for more information on molarity.
• Moles = molarity (mol.dm ${}^{-3}$ ) x volume (dm ${}^{3}$ )
• Molarity (mol.dm ${}^{-3}$ ) = $\frac{moles}{volume}$

Given the equation:

$\mathrm{NaOH}+\mathrm{HCl}\to \mathrm{NaCl}+{\mathrm{H}}_{2}\mathrm{O}$

25cm ${}^{3}$ of a sodium hydroxide solution was pipetted into a conical flask and titrated with 0.2M hydrochloric acid. Using a suitable indicator, it was found that 15cm ${}^{3}$ of acid was needed to neutralise the alkali. Calculate the molarity of the sodium hydroxide.

1. NaOH: V = 25 cm ${}^{3}$

HCl: V = 15 cm ${}^{3}$ ; C = 0.2 M

The equation is already balanced.

2. $M=\frac{n}{V}$

Therefore, n(HCl) = M $×$ V (make sure that all the units are correct!)

M = 0.2mol.dm ${}^{-3}$

V = 15cm ${}^{3}$ = 0.015dm ${}^{3}$

Therefore

$n\left(HCl\right)=0.2×0.015=0.003$

There are 0.003 moles of HCl that react

3. Look at the equation for the reaction. For every mole of HCl there is one mole of NaOH that is involved in the reaction. Therefore, if 0.003 moles of HCl react, we can conclude that the same quantity of NaOH is needed for the reaction. The number of moles of NaOH in the reaction is 0.003.

4. First convert the volume into dm ${}^{3}$ . V = 0.025 dm ${}^{3}$ . Then continue with the calculation.

$M=\frac{n}{V}=\frac{0.003}{0.025}=0.12$

The molarity of the NaOH solution is 0.12 mol.dm ${}^{3}$ or 0.12 M

4.9 g of sulfuric acid is dissolved in water and the final solution has a volume of 220 cm ${}^{3}$ . Using titration, it was found that 20 cm ${}^{3}$ of this solution was able to completely neutralise 10 cm ${}^{3}$ of a sodium hydroxide solution. Calculate the concentration of the sodium hydroxide in mol.dm ${}^{-3}$ .

1. ${\mathrm{H}}_{2}{\mathrm{SO}}_{4}+2\mathrm{NaOH}\to {\mathrm{Na}}_{2}{\mathrm{SO}}_{4}+2{\mathrm{H}}_{2}\mathrm{O}$

2. M = n/V

V = 220 cm ${}^{3}$ = 0.22 dm ${}^{3}$

$n=\frac{m}{M}=\frac{4.9g}{98g.mo{l}^{-1}}=0.05mols$

Therefore,

$M=\frac{0.05}{0.22}=0.23mol.d{m}^{-3}$
3. Remember that only 20 cm ${}^{3}$ of the sulfuric acid solution is used.

M = n/V, therefore n = M $×$ V

$n=0.23×0.02=0.0046mol$
4. According to the balanced chemical equation, the mole ratio of H ${}_{2}$ SO ${}_{4}$ to NaOH is 1:2. Therefore, the number of moles of NaOH that are neutralised is 0.0046 $×$ 2 = 0.0092 mols.

5. $M=\frac{n}{V}=\frac{0.0092}{0.01}=0.92M$

## Demonstration : the reaction of acids with carbonates

Apparatus and materials:

Small amounts of sodium carbonate and calcium carbonate (both in powder form); hydrochloric acid and sulfuric acid; retort stand; two test tubes; two rubber stoppers for the test tubes; a delivery tube; lime water. The demonstration should be set up as shown below.

#### Questions & Answers

could you please help me with the calculations of limiting reagents
Thabisa Reply
what is coulombs law
Nomhle Reply
how to calculate the relative formula for mass(m)?
Sphesihle Reply
how?
Cpheh
can you help me please
Cpheh
you add the atomic masses of the elements of the given compound
Kanelo
molecule lewis structure
diesabel Reply
Can you please explain the right hand rule for solenoid
Azafunde Reply
how do I find the percentage composition of an element
Nyiko Reply
total Mr of the element ÷total Mr of the compound all ×100
Ndimbonile
no
Globel
how to calculate distance between two charges
Kefentse Reply
how
Globel
what is a limiting reagent?
Sizwe Reply
it is a reactant that gets used up completely after a reaction
Bongy
Two charges of + 3nC and -5 nC are separated by a distance of 40cm. what is the electrostatic force between the two charges
Lethabo Reply
3,37×10^-6
Refilwe
how do you determine electron configuration?
Karabo Reply
A chemist at a water purification plant adds 700g of Al2(SO4)3 to a sample of water calculate the maximum mass of Al(OH)3 that can be produced from this mass of Al2(SO4)3
Akhonke Reply
how to calculate the mass of a sphere in ekectrostatics
Mpilo Reply
although the shape of molecules of H2S and H2O is similar,there is a remarkable difference in their boiling point explain the statement
Kgopotso Reply
What is coulomb's law
Boitumelo Reply
?
Rethabile
The amount of force between two stationary, electrically charged particles.
Olerato
Coulombs law is the magnitude of electrostatic force that one charge exerts on the other and is directly proportional to the product of the charges and inversely proportional to the squared distance between them
Refilwe
how to identify the type of intermolecular forces in pairs of molecules
Okheka Reply

### Read also:

#### Get Jobilize Job Search Mobile App in your pocket Now!

Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 11 physical science' conversation and receive update notifications? By OpenStax By OpenStax By Mahee Boo By OpenStax By Anindyo Mukhopadhyay By CB Biern By Michael Sag By OpenStax By OpenStax By Madison Christian