T3: Quantitative Chemistry

Question 1

What is the relative formula mass?

Answer 1

The relative formula mass of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Question 2

In a balanced equation, what happens with the relative formula mass?

Answer 2

The sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities.

Question 3

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

Answer 3

relative atomic mass x number of atoms of that element
————————————————————- x100
relative formula mass of the compound

Question 4

What are chemical amounts measured in?

Answer 4

Moles. The symbol for the unit mole is mol.

Question 5

What is Avogadro’s constant?

Answer 5

6.02 x 10^23, the number of particles in a mole. (the particle can be atoms, molecules or ions)

Question 6

What is one mole of a substance?

Answer 6

• One mole of any substance is just an amount of that substance that contains an Avogadro number of particles, these particles could be atoms, molecules, ions of electrons.
• One mole of a substance contains the same number of the states particles, atoms, molecules or ions as one mole of any other substance.

Question 7

What is the mass of one mole equal to?

Answer 7

The mass of one mole of a substance in grams is numerically equal to its relative formula mass (or relative atomic mass) of the element or compound.

Question 8

What is the formula to find the number of moles?

Answer 8

number of moles= mass (g) / Mr

Question 9

What is the law of conservation of mass?

Answer 9

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

Question 10

What does the law of conservation mass mean chemical reactions can be represented by?

Answer 10

chemical reactions can be represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.

Question 11

What does it mean if the mass seems to change?

Answer 11

(usually) a reactant or product is a gas which its mass hasn’t been taken into account.

Question 12

What is the explanation if the mass increases in a reaction?

Answer 12

• one of the reactants is a gas that’s found in air and all the products are solids, liquids or aqueous.
• Before the reaction, the gas is floating around in the air. It’s there but it’s not contained in the reaction vessel, so you can’t account for its mass
• When the gas reacts to form part of the product, it becomes contained inside the reaction vessel- so the total mass of the stuff inside the reaction vessel increases.

Question 13

What happens when a metal reacts with oxygen in a unsealed container?

Answer 13

The mass of the container increases. The mass of the metal oxide produced equals the total mass of the metal and the oxygen that has reacted from the air.

Question 14

What does it mean when the mass decreases in a reaction?

Answer 14

• one of the products is a gas and all the reactants are solids, liquids or aqueous.
• Before the reaction, all the reactants are contained in the reaction vessel.
• If the vessel isn’t enclosed, then the gas can escape from the reaction vessel so you can’t account for its mass- the total mass of the stuff inside the reaction vessel decreases.

Question 15

What happens in thermal decomposition of a metal carbonate in an unsealed reaction vessel?

Answer 15

• when a metal carbonate thermally decomposed to form a metal oxide and carbon dioxide gas, the mass of the reaction vessel will decrease if it isn’t sealed.
• But in reality, the mass of the metal oxide and the carbon
  dioxide produced will equal the mass of the metal carbonate that decomposed.

Question 16

How does decreased mass link to the particle model?

Answer 16

A gas will expand to fill any container it’s in. So if the reaction vessel isn’t sealed, the gas expands out from the vessel and escapes into the air around.

Question 17

What does the number in front of the chemical formulas mean?

Answer 17

how many moles of each substance takes part or is formed during the reaction.
Mg + 2HCL -> MgCl(2) + H(2)
In this reaction 1 mole of magnesium reacts with 2 moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.

Question 18

What are the steps to balance equations using the masses of reactants and products?

Answer 18

1) Divide the mass of each substance by its relative formula to find the number of moles.
2) Divide the number of moles of each substance by the smallest number of moles in the reaction.
3) If any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers.
4) Write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas.

Question 19

Why does the reaction between magnesium carbonate and hydrochloric acid eventually stop?

Answer 19

• When some magnesium carbon is placed into a beaker of hydrochloric acid, you can tell the reaction is taking place because you see lots of bubbles of gas being given off.
• after a while the amount of fizzing slows down and the reaction eventually stops.
• The reaction stops when all of one of the reactants is used up. Any other reactants are in excess.

Question 20

Why are some reactants added in excess?

Answer 20

To make sure that the other reactant is used up

Question 21

What are limiting reactants?

Answer 21

The reactant that’s completely used up in a reaction which limits the amount of products formed.

Question 22

What is the relationship between the amount of product formed to the amount of limiting reactant and why?

Answer 22

The amount of product formed is directly proportional to the amount of limiting reactant. This is because if you add more reactant there will be more reactant particles to take part in the reaction, which means more product particles.

Question 23

What are the steps to calculate the mass of the product formed in a reaction by using a mass of the limiting reactant and the balanced equation?

Answer 23

1) Write out the balanced equation.
2) Work out relative formula mass (Mr) of the reactant and product you want.
3) Find out how many moles there are of the substance you know the mass of.
4) Use the balanced equation to work out how many moles there’ll be of the other susbstance. In this case, that’s how many moles of product will be made of this many moles of reactant.
5) Use the number of moles to calculate mass.

Question 24

What is the difference between yield and theoretical yield?

Answer 24

1) yield: the mass of product of a reaction
2) theoretical yield: masses you calculate by using the mass of limiting reactants.
You never get 100% of the yield, so the amount of product you get will be less than what you calculated.

Question 25

What do equal amounts of moles of gases do?

Answer 25

At the same temperature and pressure, equal numbers of moles of any gas will occupy the same volume.

Question 26

What is the volume of one mole of any gas at room temperature and pressure (20℃ and 1 atmosphere pressure)?

Answer 26

24 dm^3

Question 27

How can you calculate the volume at gas when at room temperature and pressure?

Answer 27

volume of gas (dm^3) = (mass of gas (g) / Mr of gas) x 24

Question 28

How can you calculate volumes of gases in reactions?

Answer 28

You can use the volume of 1 gas to find the volume of another.

Question 29

What is concentration?

Answer 29

The amount of substance in a certain volume of a solution. The more solute (the substance that's dissolved) there is in a given volume, the more concentrated the solution.

Question 30

What are the 2 equations to find concentration?

Answer 30

1) concentration (g/dm^3) = mass of solute (g) / volume of solute (dm^3) 2) concentration (mol/dm^3) = number of moles of solute (mol) / volume of solvent (dm^3)

Question 31

When can concentration be found?

Answer 31

Titration calculations. If the volumes of two solutions that react completely are known, the concentration of the other solution can be calculated. (the concentration can be converted from mol/dm^3 to g/dm^3)

Question 32

What is the atom economy (atom utilisation)?

Answer 32

A measure of the amount of starting materials end up as useful products, and how much is wasted when manufacturing a chemical.

Question 33

How can you calculate atom economy?

Answer 33

atom economy= (relative formula mass of desired products / relative formula mass of all reactants) x 100

Question 34

What does an atom economy of 100% mean?

Answer 34

all the atoms in the reactants have been turned into useful/desired products. The higher the atom economy the 'greener' the process.

Question 35

What are the disadvantages of low atom economy?

Answer 35

- Use up resources very quickly. They make lots of waster materials that have to be disposed of somehow. That tends to make these reactions unsustainable, the raw materials will run out and the waster has to go somewhere. - low atom economy reactions aren't usually profitable. Raw materials are often expensive to buy, and waste products can be expensive to remove and dispose of responsibly.

Question 36

How can the problems of low atom economy be avoided?

Answer 36

Find a use for the waste products rather than just throwing them away. There's often more than one way to make the product you want, so the trick is to come up with a reaction that gives useful "by product" rather than useless ones.

Question 37

What are the factors that has to be considered when choosing which reaction pathway is chosen to produce a specified product?

Answer 37

the yield, the rate of the reaction, equilibrium position and usefulness of by products.

Question 38

How can you calculate the percentage yield?

Answer 38

percentage yield = (mass of product actually made (g) / maximum theoretical mass of product) x 100

Question 39

What is the percentage yield always somewhere between?

Answer 39

0% to 100%. 100% yield means that you got all the product to get. 0% yield means that no reactants were converted into product, no product was made at all.

Question 40

Why should industrial processes have as high a percentage yield as possible?

Answer 40

to reduce waste and to reduce cost

Question 41

Why is it not always possible to obtain the calculated amount of a product?

Answer 41

- the reaction may not always go to completion - some of the product may be lost when it is separated from the reaction mixture - some of the reactants may react in ways different to the expected reaction

Question 42

How does reactions not always going to completion means?

Answer 42

In reversible reactions, the products can turn back into reactants, so the yield will never be 100%.

Question 43

How does some of the product being lost when it is separated from the reaction mixture affect the yield?

Answer 43

1) When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or a bit of solid. -If you want to keep the liquid, you'll lose the bit that remains with the solid and filter paper (as they always stay a bit wet). - If you want to keep the solid, some of it'll get left behind when you scrape it off the filter paper. 2) You'll also lose a bit of material when you transfer it from one container to another- even if you manage not to spill it. Some of it always gets left behind on the inside surface of the old container.

Question 44

How does some of the reactants reacting in ways different to the expected reaction?

Answer 44

There might be side reactions. The reactants sometimes react differently to how you expect. They might react with gases in the air or impurities in the reaction mixture, so they end up forming extra products other than the ones you want.

Question 45

Why do some reactions have a higher atom economy?

Answer 45

- Reactions with higher atom economy are the ones that only have one products. Those reactions have an atom economy of 100%. - The more products there are, the lower the atom economy is likely to be.

Question 46

What happens when you increase the mass of a solute and keep the volume the same?

Answer 46

The concentration increases.

Question 47

What happens if you increase the volume of solution and keep the mass of the solute the same?

Answer 47

Then the concentration decreases.