Ardingly - Remove - Moles lesson 2

Moles Lesson 2

Recap - Main points from lesson 1

• A mole is just a counting unit for particles, just like a dozen is a counting unit for eggs.

• The mass of one mole of any atom is the same as its relative mass (from the periodic table) in grams.

• The relative molecular mass (M_r) of a compound is the sum of the relative atomic mass of all the atoms in that compound.

• To convert between mass, moles and Mr, use the moles triangle.

Multiple Choice

How many moles of magnesium in a 20g strip?

• 0.81

• 0.82

• 0.83

• 0.84

Multiple Choice

How many moles of water in a 250ml glass?

Clue: 1ml=1g

• 13.4

• 13.8

• 14.0

• 15.2

Multiple Choice

What is the molar mass of calcium hydroxide, Ca(OH)₂?

• 74 g

• 57 g

• 58 g

• 38 g

Multiple Choice

Which has more molecules?

• 1 mole H₂O
• 1 mole Al(OH)₃
• 1 mole NaCl
• There are all the same

Multiple Choice

What is the mass of one mole of sulphur?

• 10 g

• 12 g

• 32 g

• 44 g

Multiple Choice

Which of the following would have the highest number of moles?

• 4g CO₂
• 15g H₂O
• 22g NaCl
• 100g AgCl

Multiple Choice

Which of the following would have more mass?

• 1 mole of Li
• 1 mole of Au
• 1 mole of Si
• None, all are equal

Multiple Choice

0.5 moles of an unknown substance has a mass of 22g. Calculate the Mr.

• 11

• 44

• 32

• 66

Moles - lesson 2

Part 2 - percentage by mass

In this lesson we will learn how to calculate the percentage by mass of a particular element in a compound

% by mass - Worked example 1

Q. what is the percentage by mass of chlorine in magnesium chloride - Mg Cl₂ ?

% by mass - Worked example 2

Q. what is the percentage by mass of nitrogen in ammonium nitrate- NH₄NO₃ ?

Multiple Choice

What is the general formula for the computation of percentage composition of the given element?

• % mass of element = mass of element in the compound divided by mass of the compound times 100

• % mass of element = mass of element in the compound divided by mass of the compound

• % mass of element = mass of element in the compound divided by 100

• % mass of element = mass of element in the compound

Multiple Choice

What is the percent by mass of chlorine in NaCl?

• 39%

• 61%

• 35%

• 35%

Multiple Choice

What is the percent by mass of magnesium in MgO?

• 20%

• 40%

• 50%

• 60%

Multiple Choice

What is the percent by mass of fluorine in CaF₂ (gram-formula mass = 78 g/mol)?

• 24%

• 49%

• 49%

• 65%

Multiple Choice

What is the percent by mass of calcium in CaF₂ (gram-formula mass = 78 g/mol)?

• 24%

• 49%

• 51%

• 65%

Multiple Choice

Percentage composition of a compound tells you the percentage of the mass made up by each element in a compound.

• TRUE

• FALSE

Multiple Choice

Find the percent composition of Cu₂S?

• %Cu= 67.987 %S= 32.013

• %Cu= 79.854 %S= 20.145

• %Cu= 35.946 %S= 64.054

Multiple Choice

Which element in the compound potassium cyanide, KCN, has a percent composition of 60.97%?

• K

• C

• N

• None

Multiple Choice

How many percent of carbon is present in chloroform?

• 10.06%

• 0.84%

• 89.09%

• 99.99%

Multiple Choice

How many percent of chlorine is present in chloroform?

• 10.06%

• 0.84%

• 89.09%

• 99.99%

Multiple Choice

Find the percentage composition of Mg in Mg₃(PO₄)₂.

• 27.48% Mg

• 43.11% Mg

• 16.00% Mg

• 12.63% Mg

Multiple Choice

What is the percentage of oxygen in carbon dioxide? (CO2)

• 27.3%
• 72.7%
• 30%
• 70%

Multiple Choice

What is the percentage of OH^– in Ca(OH)₂?

• 45.9%

• 75%

• 66.6%

• 90.1%