Quantitative Chemistry C3

Question 1

What is relative formula mass?

Answer 1

Relative formula mass (M_r) is the sum of the relative atomic masses of all the atoms in a molecular formula.

Question 2

How do you find the relative formula mass of MgCl2?

Answer 2

1) Look up the relative atomic masses: A of Mg = 24, A of Cl = 35.5.
2) Calculate: M_r = 24 + (2 x 35.5) = 95.

Question 3

How do you calculate the percentage mass of an element in a compound?

Answer 3

Percentage mass = (A x number of atoms of that element / M_r of the compound) x 100.

Question 4

What is the percentage mass of sodium in sodium carbonate (Na2CO3)?

Answer 4

M_r of Na2CO3 = (2 x 23) + 12 + (3 x 16) = 106.
Percentage mass of sodium = (23 x 2 / 106) x 100.

Question 5

How do you find the mass of iron in a mixture containing 20% iron ions?

Answer 5

In a 50 g mixture, the mass of iron = 50 x 0.20 = 10 g.

Question 6

How do you calculate the percentage mass of iron in iron chloride (FeCl3)?

Answer 6

M_r of FeCl3 = 56 + (3 x 35.5) = 142.5.
Percentage mass of iron = (56 / 142.5) x 100 = 39.44%.

Question 7

What is the relative atomic mass (A) of an element?

Answer 7

The relative atomic mass is the same as its mass number and can be found on the periodic table.

Question 8

How much iron chloride is needed to provide 10 g of iron in a mixture?

Answer 8

To provide 10 g of iron, you need to calculate the mass of iron chloride based on its percentage mass of iron.

Question 9

What is the percentage composition by mass of potassium in potassium hydroxide (KOH)?

Answer 9

Calculate using the formula: (A of K / M_r of KOH) x 100.

Question 10

What is a mole?

Answer 10

A mole is simply the name given to an amount of a substance, similar to how ‘a million’ represents 1,000,000.

The Avogadro constant is 6.02 x 10^23.

Question 11

How many particles are in one mole?

Answer 11

One mole of any substance contains Avogadro’s number of particles, which is 6.02 x 10^23. The particles could be atoms, molecules, ions, or electrons.

Question 12

What is the relationship between moles and mass?

Answer 12

The mass of one mole of atoms or molecules of any substance is equal to the relative atomic mass (A_r) or relative formula mass (M_r) of that substance in grams.

Question 13

What is the mass of one mole of carbon?

Answer 13

One mole of carbon has a relative atomic mass (A_r) of 12 g.

Question 14

What is the mass of one mole of nitrogen gas (N2)?

Answer 14

One mole of nitrogen gas (N2) has a relative formula mass (M_r) of 28 g (2 x 14).

Question 15

What is the mass of one mole of carbon dioxide (CO2)?

Answer 15

One mole of carbon dioxide (CO2) has a relative formula mass (M_r) of 44 g (12 + [2 x 16]).

Question 16

How do you calculate the number of moles from mass?

Answer 16

Number of moles = mass in grams (of an element or compound) / M_r (of the element or compound).

Question 17

How many moles are in 66 g of carbon dioxide (CO2)?

Answer 17

To find the number of moles in 66 g of CO2, use the formula: 66 g / 44 g/mol = 1.5 mol.

Question 18

What is the mass of 4 moles of carbon?

Answer 18

The mass of 4 moles of carbon is 4 x 12 g = 48 g.

Question 19

What is the formula triangle for calculating moles?

Answer 19

The formula triangle is: M = mass / number of moles. Cover the variable you want to find.

Question 20

What is a common misconception about moles?

Answer 20

Calculations involving moles can be confusing, but they don’t need to be. Understanding the basics can help alleviate the confusion.

Question 21

What is the Conservation of Mass?

Answer 21

You can’t create or destroy matter; mass is always conserved in chemical reactions.

Question 22

What happens to atoms during a chemical reaction?

Answer 22

No atoms are destroyed and no atoms are created.

Question 23

What can be said about the number and types of atoms in a reaction equation?

Answer 23

The same number and types of atoms are present on each side of a reaction equation.

Question 24

What does it mean when mass is conserved?

Answer 24

No mass is lost or gained during a chemical reaction.

Question 25

How can you show that mass is conserved in a reaction?

Answer 25

By adding up the relative formula masses of the substances on each side of a balanced equation.

Question 26

Example of mass conservation in a reaction: 2Li + F2 → 2LiF.

Answer 26

Total mass of reactants equals total mass of products: 52 = 52.

Question 27

What happens if the mass seems to change during a reaction?

Answer 27

Usually, a gas is involved.

Question 28

What does it indicate if the mass increases during a reaction?

Answer 28

It likely means one of the reactants is a gas from the air.

Question 29

What happens when a gas reacts and is contained in a reaction vessel?

Answer 29

The total mass inside the vessel increases as the gas is now accounted for.

Question 30

What does it indicate if the mass decreases during a reaction?

Answer 30

It likely means one of the products is a gas that escapes the reaction vessel.

Question 31

What happens when a metal carbonate decomposes?

Answer 31

The mass of the reaction vessel decreases if it's not sealed, but the total mass of products equals the mass of the reactants.

Question 32

What is a key takeaway about mass in reactions?

Answer 32

The total mass of reactants is always equal to the total mass of products.

Question 33

What is concentration in chemistry?

Answer 33

Concentration is the amount of a substance (e.g. mass or number of moles) in a certain volume of a solution.

Question 34

How does the amount of solute affect concentration?

Answer 34

The more solute there is in a given volume, the more concentrated the solution.

Question 35

What are the units for measuring concentration?

Answer 35

Concentration can be measured in g/dm³.

Question 36

How do you calculate concentration in g/dm³?

Answer 36

Concentration = mass of solute / volume of solvent in dm³.

Question 37

What is the concentration of a solution with 30 g of sodium chloride in 0.2 dm³ of water?

Answer 37

The concentration is 150 g/dm³.

Question 38

How do you convert cm³ to dm³?

Answer 38

1 dm³ = 1000 cm³.

Question 39

What is the concentration of a solution with 15 g of salt in 500 cm³?

Answer 39

First, convert 500 cm³ to dm³: 500 cm³ = 0.5 dm³. Then, concentration = 15 g / 0.5 dm³ = 30 g/dm³.

Question 40

How can you find the mass of solute if you know its concentration?

Answer 40

Use the formula: mass = concentration x volume.

Question 41

What mass of salt is needed to create a solution with a concentration of 24 g/dm³ in 0.40 dm³ of water?

Answer 41

mass = 24 g/dm³ x 0.40 dm³ = 9.6 g.

Question 42

What should you remember about measurements?

Answer 42

All measurements have some uncertainty.