The Chemistry CLASS with Nelson Etafo

18/06/2025

What is the half-life of a radioactive isotope if a 500.0g sample decays to 62.5g in 24.3 hours?

02/02/2025

Question

What is Avogadros law?

22/08/2024

What is separating technique?

This is a platform to demystify chemistry to the high school students to become excellent.

08/07/2024

Chemistry question

0.16g of methane when burnt increases the temperature of 100g of water by 400°C, what is the heat of combustion of methane if the heat capacity of water is 4.2 Jg-¹°C-¹? (CH4=16).
(A) 1,160kJmol-¹
(B) 1,180kJmol-¹
(C) 1,560kJmol-¹
(D) 1,600kJmol-¹
(E) 1,680kJmol-¹

08/07/2024

Chemistry question

The pollutants that are likely to be present in an industrial environment are
(A) H2S, SO2and oxides of nitrogen
(B)NH3, HCl and CO
(C) CO2, NH3, and H2S
(D) dust, NO and Cl2

08/07/2024

What is radioactivity?

14/03/2024

0.16g of methane when burnt increases the temperature of 100g of water by 400°C, what is the heat of combustion of methane if the heat capacity of water is 4.2 Jg-¹°C-¹? (CH4=16).
(A) 1,160kJmol-¹
(B) 1,180kJmol-¹
(C) 1,560kJmol-¹
(D) 1,600kJmol-¹
(E) 1,680kJmol-¹

14/03/2024

Question

The partial pressure of oxygen in a sample of air is 500 mmHg. If the total Pr pressure is 780 mmHg, what is the mole fraction of the oxygen?

14/03/2024

Question

Chlorine, consisting of two isotopes of mass numbers 35 and 37, has an atomic mass of 35.5.
The relative abundance of the isotope of mass number 37 is ___

03/03/2024

Question

200 cm³;of air was passed over heated copper in a syringe several times to produce copper (II) oxide. When cooled, the final volume of air recorded was 158 cm³. Estimate the percentage of oxygen in the air.

11/02/2024

Hello kindly engage on the exercises on The Chemistry CLASS with Nelson Etafo

05/02/2024

Mole Concept X

Further calculations

Today we are learning how to determine the unknown concentration of a solution with known volume and number of moles.

Formula

Mole =
concentration (mol/dm3) x volume (dm3)

Or

Mole =
concentration (mol/dm3) x volume (cm3)/ 1000

Example

Calculate the concentration present in

a) 0.125 mol of 2.5 dm3 of NaOH solution
b) 17.6g of 470 cm3 of HCl solution
c) 7.48 x 10^20 ions of 700 cm3 of CaCl2 solutions

(H = 1, Cl = 35.5, L = 6.02 x 10^23)

Solution

a) First write down your known and unknown data

Concentration = y

Volume = 2.5 dm3

mole = 0.125 mol

Mole =
concentration (mol/dm3) x volume (dm3)

0.125 = y x 2.5

y = 0.125 /2.5
= 0.0500 mol/dm3

Note my answer is written to 3 s.f.

b) First write down your known and unknown data

Concentration = y

Mass = 17.6 g

Volume = 470 cm3
= 470 / 1000 = 0.470 dm3

(Let's convert to dm3

1 dm3 = 1000 cm3)

mole = ?

Molar mass of HCl = 1 + 35.5 = 36.5 g/mol

mole = given mass/ molar mass
= 17.6 / 36.5
= 0.4822
= 0.482 mol

Mole =
concentration (mol/dm3) x volume (dm3)

0.482 = y x 0.470

y = 0.482 / 0.470
= 1.03 mol/dm3

Note my answer is written to 3 s.f

c) First write down your known and unknown data

Concentration = y

Number of ions = 7.48 x 10^20 ions

Volume = 700 cm3

mole = ?

Number of molecules
= moles x Avogadros constant

7.48 x 10^20 ions = moles x 6.02 x 10^23

Moles = (7.48 x 10^20)/(6.02 x 10^23)
= 0.00124 moles

Mole =
concentration (mol/dm3) x volume (cm3)/1000

0.00124 = y x 700/1000

y = 0.00124 × 1000/700
= 0.00177 mol/dm3

Note my answer is written to 3 s.f

Exercise

Calculate the concentration present in

a) 0.576 mol of 0.25 dm3 of NaCl solution
b) 34.6g of 800 cm3 of KOH solution
c) 5.49 x 10^21 ions of 500 dm3 of CaCl2

(K = 39, O = 16, H = 1, L = 6.02 x 10^23)