Quantitative Chemistry

Question 1

What is the law of conservation of mass?

Answer 1

No atoms are lost or made during a chemical reaction so the mass of products equals the mass of the reactants.

Question 2

How can the overall charge of zero form in an ionic compound?

Answer 2

The charges on the ions have to cancel out to leave an overall charge of zero.

Question 3

What is the relative atomic mass (Ar)?

Answer 3

Its the average mass of the isotopes of that element weighted to take into account the abundance of each isotope.

Question 4

What is the relative formula mass (Mr)?

Answer 4

Its the sum of the Relative Atomic Masses of the atoms in the numbers shown in the formula and has no units and never involves big numbers.

Question 5

What is the Formula for Percentage by mass of an element?

Answer 5

Total RAM of the atoms of that element
——————————————————- x 100
Relative formula mass (Mr) of compound

Question 6

How do we calculate the number of moles in an element?

Answer 6

Mass (g) / RAM (Ar)

Question 7

How do we calculate the number of moles in a compound?

Answer 7

Mass (g)
———— = Number of moles in a compound
RFM (Mr)

Question 8

What is the number for Avogadro’s constant?

Answer 8

6.02 x 10²³

Question 9

What is it meant by a limiting reactant?

Answer 9

A reactant that is fully used up

Question 10

What is it meant by the excess reactant?

Answer 10

The reactant that is not used up

Question 11

What is it meant by concentration?

Answer 11

Tells us the mass of a solute in a given volume of solution or the number of moles of a solute in a given volume of solution.

Question 12

What is it meant by a solute?

Answer 12

A chemical that is dissolved in a solvent.

Question 13

What is concentration measured in?

Answer 13

g / dm³ and mol / dm³

Question 14

How do we calculate concentration?

Answer 14

Mass (g)
——————— = Concentration (g / dm³)
Volume (dm³)

Question 15

How can we increase the concentration in a solution?

Answer 15

If we increase the mass of the solute and keep the volume the same, then we increase the concentration.

Question 16

How can we decrease the concentration in a solution?

Answer 16

If we increase the volume of the solution and keep the mass of the solute the same, then we decrease the concentration.

Question 17

(Triple only) Why is it not always possible to achieve 100% yield in a chemical reaction?

Answer 17

-Some of the product may be lost when it is separated from the reaction mixture.
-Some of the reactants may react in different ways to the expected reaction so we do not get the product we expect.
-Reversible reactions may not go to completion.

Question 18

(Triple only) What is the equation for percentage yield?

Answer 18

Mass of product actually made
——————————————————– x 100 = % Yield
Maximum theoretical mass of product

Question 19

(Triple only) Why can we not achieve greater than 100% yield?

Answer 19

This would mean that we had created atoms which due to the law of conservation of mass we cannot create atoms.

Question 20

(Triple only) What is Atom economy?

Answer 20

Its a measure of the amount of starting materials that end up as useful products.

Question 21

(Triple only) What are the advantages of Atom Economy?

Answer 21

-By minimising the production of unwanted products, we save money.
-We also increase sustainability by not wasting resources.

Question 22

(Triple only) What is the equation for atom economy?

Answer 22

RFM of desired products
—————————————— x 100 = Atom Economy
Sum of RFM of all reactants

Question 23

(Triple only) How do chemists use unwanted side-products to save money in atom economy?

Answer 23

They try to find a use for the side-products for example in other reactions.

Question 24

(Triple only) How much does a gas occupie a volume?

Answer 24

24 dm³

Question 25

(Triple only) What is the equation for gas volumes?

Answer 25

Number of moles x 24 = Volume (dm³)