Topic 3-Quantitative chemistry

Question 1

What is the relative formula mass of a compound?

Answer 1

the sum of all the relative atomic masses of the atoms in the numbers shown is the formula

Question 2

What is the symbol for the unit mole?

Answer 2

mol

Question 3

What is one mole of any substance?

Answer 3

an amount of that substance that contains an Avogadro number of particles
- these particles could be atoms, molecules , ions or electrons

Question 4

Carbon has a relative atomic mass of 12
- What does one mole of carbon weigh?

Answer 4

exactly 12g

Question 5

Nitrogen gas has a relative formula mass of 28 (2 * 14)
What does one mole of nitrogen gas weigh?

Answer 5

exactly 28g

Question 6

Carbon dioxide has a relative formula mass of 44
- What does one mole of carbon dioxide weigh?

Answer 6

44g

Question 7

Carbon has a relative atomic mass of 12?
- How many particles are in 12g of carbon?

Answer 7

6.02*10 to the power of 23
‘the Avogadro constant’

Question 8

Nitrogen gas has a relative formula mass of 28
- How many particles are in 28g of nitrogen gas?

Answer 8

‘The Avogadro constant’

Question 9

How is mass conserved during a reaction?

Answer 9

• no atoms are destroyed or created
• there are the same number of each type of atom on both sides if the equation
• so no mass is lost or gained

Question 10

Why may the mass appear to change in a reaction?

Answer 10

When there is a gas in the reactants or products
- and the reaction takes place in an unsealed reaction vessel

Question 11

Why may the mass appear to increase in a reaction?

Answer 11

• one of the reactants is a gas that is found in the air (e.g. oxygen)
• all the products are solids, liquids or aqueous

Question 12

Explain why one of the reactants being a gas may make the mass appear to increase (during the reaction)?

Answer 12

• Before the reaction:
  The gas is floating around in the air
  It is not contained in the reaction vessel so its mass can’t be accounted for
• After the reaction:
  The gas reacts to form part of the product
  It becomes contained in the reaction vessel
  So its mass is now accounted for meaning the mass of substance in the reaction vessel increases

Question 13

Give an example of when the mass of a reaction may appear to increase?

Answer 13

• When a metal reacts with oxygen in an unsealed container, the mass of the container increases
• the mass of metal oxide produced equals the total mass on the metal and the oxygen that reacted from the air

Question 14

Why may the mass appear to decrease in a reaction?

Answer 14

• one of the products is a gas
• all the reactants are solid, liquid or aqueous

Question 15

Explain why one of the products being a gas may make the mass appear to decrease (during the reaction)?

Answer 15

Before the reaction:
- all the reactants are contained in the reaction vessel and their mass is accounted for
After the reaction:
- If the vessel isn’t enclosed then the gas would have escaped as it is formed
- its mass is no long er accounted for
- the total mas of substance inside the reaction vessel decreases

Question 16

Give an example of when the mass of a reaction may appear to increase?

Answer 16

• When a metal carbonate thermally decomposes to form a metal oxide and carbon dioxide gas
• the mass of the reaction vessel will decrease if it isn’t sealed
• however, the mass of metal oxide and the carbon dioxide produced will equal the mass of the metal carbonate that decomposed

Question 17

What do the ‘big numbers’ before the chemical formulas of the reactants or products tell you?

Answer 17

How many moles of each substance takes part or is formed during the reaction

Question 18

What does the atom economy (or atom utilistation) of a reaction tell you?

Answer 18

How much of the mass of the reactants is wasted when manufacturing a chemical and how much ends up as useful products

Question 19

What does it mean if a reaction has 100% atom economy?

Answer 19

all of the atoms in the reactants have been turned into useful (desired) products

Question 20

The higher the atom economy, the

Answer 20

‘greener’ the process

Question 21

Explain the issues with a reaction having a low atom economy?

Answer 21

-use up resouces very quickly
- they make lots of waste that has to be disposed of (this makes these reactions unsustainable)
- usually aren’t profitable (it is expensive to but raw materials and then disposing and removing waste products responsibly)

Question 21

What is he best way to get around the issue of a reaction having a low atom economy?

Answer 21

• find a use for the waste products rather than just throwing them away
• there is often more than one way to make the product you want, so a better reaction would have useful ‘by-products’

Question 21

What reactions have the highest atom economy?

Answer 21

• the ones that only have one product
  these have an atom economy of 100%

Question 21

the more products there are…

Answer 21

the lower the atom economy is likely to be

Question 21

What other factors ( as well as atom economy) need to be considered when choosing what reaction to use to make a certain product?

Answer 21

-the yield
-rate of reaction
-the postion of equilibrium for reversible reactions