Topic 4 Calculations

Question 1

What is meant by RFM?

Answer 1

Relative formula mass

Question 2

How do you calculate RFM?

Answer 2

Add the atomic mass of all of the atoms within the compound together

Question 3

How do you find out the atomic mass of an element?

Answer 3

Use the bottom number of the element on the periodic table

Question 4

What is meant by the term conservation of mass?

Answer 4

Mass cannot be created or destroyed during a reaction. Therefore the mass of reactants will always equal the mass of the products.

Question 5

How does a balanced formula equation for a reaction show that mass is conserved during a reaction?

Answer 5

There is the same number of each element on each side of the equation

Question 6

When balancing equations where is the only place that you can place a number, and what does this number do?

Answer 6

Infront of a compound/element and it acts as a multiplyer of the compound/element.

Question 7

A scientist measures the mass of a lump of metal in an open container, then forgets to put it away for before going on holiday for a number of weeks. When they return and measure the mass of the metal, the reading increased, suggest why the reading has increased.

Answer 7

The metal must have reacted with a gas , therefore adding mass.

Question 8

The equation for the reaction between sulfuric acid and sodium carbonate is: H2SO4 (aq) + Na2CO3(aq) ? Na2SO4(aq) + H2O(l) + CO2(g) This reaction is carried out in an open beaker on a balance. During the reaction the mass decreases, explain why the mass decreases.

Answer 8

The mass decreases as a gas is given off. This carbon dioxide will escape the container.

Question 9

What is meant by Mr?

Answer 9

Mr means Molecular mass, which is the same as RFM

Question 10

True or False: The Mr of a compound is always greater than the Ar of any of the element sin in the compound?

Answer 10

True, the Mr is found by adding up the Ar of each atom within the compound, so it must be greater.

Question 11

What is meant by Ar?

Answer 11

Atomic Mass

Question 12

The Ar of O=16 and the Ar of N=14. What is the Mr of NO?

Answer 12

Mr = 30. (1x14) + (1x16)

Question 13

The Ar of F=19 and AR of S=32. How would you calculate the Mr of SF6?

Answer 13

Mr = (1x19) + (6x32) = 146

Question 14

The Ar of Be=9, O=16 and H=1. How would you calculate the Mr of Be(OH)2?

Answer 14

Mr = (1x9) + (2x16) + (2x1) = 43. Remember the small number outside the bracket means everything inside the bracket is multiplied by that number.

Question 15

The molecular formula of hexane is C6H14. What is the empirical formula?

Answer 15

The largest number that goes into both 6 and 14 is 2. Divide both numbers by 2 to give you your empirical formula of C3H7.

Question 16

What is the definition of a mole?

Answer 16

One mole is 6.02x10^23 particles of a substance.

Question 17

How is the Ar and Mr of a substance is related to the idea of moles?

Answer 17

The value of Ar of an element and Mr of a compound is equal to the mass in grams of 1 mole of the substance. E.g. CO2 Mr=44, 1 mole of CO2 = 44g

Question 18

What contain more particles, a mole of water or a mole of oxygen?

Answer 18

The both contain the same number of particles.

Question 19

How do you calculate moles?

Answer 19

Moles=mass/Mr

Question 20

How do you calculate mass, given the nuber of moles?

Answer 20

Mass=moles x Mr

Question 21

How do you calculate Mr, if given mass and moles?

Answer 21

Mr= mass/moles

Question 22

3 moles of compound A have a mass of 126g. What is the Mr of compound A?

Answer 22

Mr= mass/moles. Therefore Mr = 126/3=42

Question 23

How many particles in 0.5 moles of water?

Answer 23

0.5 x 6.02x10^23 = 3.01x10^23

Question 24

How do you go from cm3 to dm3?

Answer 24

divide by 1000

Question 25

How do you go from dm3 to cm3?

Answer 25

multiply by 1000

Question 26

How do you find the concentration of a solution in g/dm3?

Answer 26

concentration (g/dm3) = mass(g)/volume(dm3)

Question 27

A bottle that holds 2dm3 of drink has 130g of sugar in. What is the concentration of sugar in the drink in g/dm3?

Answer 27

Concentration = 130g/2dm3 = 65g/dm3

Question 28

How do you calculate mass given concentration and volume?

Answer 28

Mass (g) = concentration (g/dm3) x Volume (dm3)

Question 29

What does empirical formula tell you?

Answer 29

The empirical formula tells you the smallest hole number ratio of atoms in a compound.

Question 30

How can an empirical formula be worked out from the masses of the elements in a sample of a compound?

Answer 30

1.Divide the mass by the relative atomic mass of each element. 2.Divide these answers by the smallest of the answers. 3.Work out your whole number ratio and write out your compound.

Question 31

Describe the experiment needed to find the empirical formula of magnesium oxide, using magnesium ribbon and a crucible.

Answer 31

Measure the mass of magnesium ribbon in a crucible (this will give you the mass of magnesium). Then heat in a crucible lifting the lid slightly every minute. One heated for 10 minutes, leave to cool and reweigh (this will give you the mass of magnesium oxide). Subtract the magnesium ribbon mass from the magnesium oxide mass to calculate the mass of oxygen.

Question 32

What does it mean if a reactant is in excess?

Answer 32

An excess is if there is more than enough, e.g. a reactant is in excess if there is more of it present than the limiting reactant to be used up.

Question 33

Why is the mass of a productfromed controlled by the reactant not in excess?

Answer 33

The reaction stops when this reactant is used up, so no more product is made.

Question 34

When making copper sulphate using copper oxide and sulfuric acid, what was added in excess?

Answer 34

Copper oxide

Question 35

0.8g of oxygen reacts sompletely with 3.9g of potassium to from potassium oxide. Work out the mass of potassium oxide made in this reaction.

Answer 35

4.7g