Aug 12, 2014 - Molar Volume Vm ... Vm = molar volume = 22,4 dm3) ... A balanced equation tells us reactants and the prod
MOLE CALCULATIONS
12 AUGUST 2014
Lesson Description In this lesson we:
Introduce the concept of a mole. Discuss Avogadro’s number and molar mass. Consider the information given in a balanced equation. Solve problems with moles and mass.
Summary The Mole Concept The mole (abbreviation “mol”) is the SI unit for “amount of substance”. One mole of any substance is the same number of atoms found in 12g of carbon-12.
Avogadro’s Number NA 23
The number of atoms in one mole of carbon-12. NA = 6,022 × 10 To simplify the relationship into an equation:
n = number of moles N = number of particles of sample NA = Avogadro’s number = 6,022 × 10
23
particles, molecules or atoms
Molar mass Molar mass (M) is the mass of 1 mole of a chemical substance. The unit for molar mass is grams per −1 mole or g·mol . To simplify the relationship into an equation:
n = number of moles m = mass of sample (g) -1
M = Molar mass (g.mol )
Molar Volume Vm The volume of one mole of any gas at standard temperature and pressure (STP) is equal to 22,4 dm 0 Standard Pressure = 101,3 kPa and Standard Temperature = 273 K =0 C To simplify the relationship into an equation:
n = number of moles 3
V = volume of sample (dm ) 3
Vm = molar volume = 22,4 dm )
3
Concentration In a solution, the number of moles of solute divided by the volume of solution gives the concentration of the solution. To simplify the relationship into an equation:
-3
C = concentration (mol.dm ) n = number of moles of solute 3
V = volume of solution (dm ) A balanced equation tells us reactants and the products and the ratio in which they react. The number in front of the chemical symbols describes the mole ratio of the reactants and products
Empirical and Molecular Formulae Empirical formula is the simplest ratio of elements in a compound. Molecular formula is the ratio of atoms in the molecule of a compound.
Test Yourself Question 1 3
How many moles of carbon dioxide are present in 5,6dm at STP? A
3
B
1
C
0,5
D
0,25
Question 2 3
How many moles of oxygen atoms are present in a 5,6 dm sample of carbon dioxide at STP? A
0,25
B
0,5
C
1
D
2
Question 3 -3
What is the concentration of sodium ions in a 0,5mol.dm of sodium sulphate? -3
A
0,5 mol.dm
B
0,25 mol.dm
C
1,5 mol.dm
D
1 mol.dm
-3
-3
-3
Question 4 3
-3
How many sodium ions are present in 5cm of a 0,5mol.dm sodium sulphate solution? 21
A
3 x 10
B
3 x 10
C
1,5 x 10
D
1,5 x 10
22 21
22
Question 5 What is the percentage of potasium in a sample of potassium oxide? A
83%
B
55%
C
41,5%
D
14,2%
Improve your Skills Question 1 A Helium balloon contains 2,4g of helium gas at STP. 1.1
How many moles of helium are there in the balloon?
1.2
What is the volume of the balloon?
1.3
How many helium molecules are present in the balloon?
Question 2 3
A 250 cm aqueous solution of sodium chloride is made using 5,8g of salt 2.1
What is the concentration of the solution?
2.2
How many ions of sodium are present in the solution?
2.3
How many moles of water are present?
2.4
What is the ratio between the number of water molecules and chloride ions?
Question 3 A sample of water taken from the Middle Oliphants River basin contains a compound made from nitrogen, hydrogen and oxygen. The percentage composition is % Nitrogen = 35% % Hydrogen = 5% 3.1 3.2
Find the empirical formula of the compound Suggest a possible molecular formula
Links
Enter the South Africa Water Game Challenge 2014. Go to www.watergame.co.za and register and to get more details. There are great prizes to be won too!
