TOPIC 3 - QUANTITATIVE CHEMISTRY

Question 1

What is relative formula mass?

Answer 1

All of the masses of the atoms in the molecular formula added together.

Question 2

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

Answer 2

Percentage mass of an element in a compound = (relative atomic mass x number of atoms of that element / relative formula mass of the compound) x 100

Question 3

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

Answer 3

Percentage mass of an element in a compound = (atomic mass x number of atoms in that element) / relative formula mass of the compound
X100

Question 4

What is the Avogadro constant?

Answer 4

6.02 x 10*23

Question 5

What is one mole of a substance?

Answer 5

One mole of any substance is just an amount of that substance that contains an Avogadro number of particles. So, 6.02 x10*23 particles.

Question 6

What is the relationship between moles and relative formula mass?

Answer 6

One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass for that substance.
Eg carbon has an Ar of 12. So one mole of carbon weighs exactly 12 grams.

Question 7

What is the formula for the number of moles in a given mass?

Answer 7

Number of moles = mass in g (of an element or compound) / Mr (of the element or compound.

Question 8

What is one explanation for a change of mass during a reaction? (Increases)

Answer 8

If the mass increases, it’s probably because one of the reactants is a gas that is found in the air eg oxygen and all the products are solids, liquids or aqueous.
Before the reaction, the gas is floating around in the air. It is there but it isn’t 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 9

What is one explanation for a change of mass during a reaction? (Decreasing)

Answer 9

If the mass decreases, it’s probably because one of the products is a Gaza small the reactants are solids, liquids or aqueous.
Before the reaction, all the reactants are contained in the reaction vessel.
If the vessel is t enclosed, then the gas can escape from the reaction vessel as it is formed. It’s no longer contained in the reaction, so you cannot account for its mass. The total mass of the stuff inside the reaction vessel decreases.

Question 10

When do reactions stop?

Answer 10

When one of the reactants is used up (the limiting reactant).
The amount of products formed is directly proportional to the amount of the limiting reactant. For example, if you half the amount of the limiting reactant the amount of product formed will also half. If you double the amount of the limiting reactant the amount of product will double (as long as it is still the limiting reactant).

Question 11

What is the formula to find the volume of gas?

Answer 11

Volume of gas = mass of gas (g) / Mr of gas

X100

Question 12

What volume does one mole of any gas occupy?

Answer 12

24 dm*3.

Question 13

What is the concentration of a solution?

Answer 13

The amount of the substance in a certain volume of a solution is called its concentration.
The more solute there is in a given volume, the more concentrated the solution.

Question 14

How do you calculate concentration in g/dm*3?

Answer 14

Concentration (g/dm3) = mass of solute in grams / volume of solvent in dm3.

Question 15

How do you calculate the concentration of a solution in mol/dm*3?

Answer 15

Concentration (mol/dm3) = number of moles of solute in mol / volume of solvent in dm3.

Question 16

How can you calculate mol/dm3 into g/dm3?

Answer 16

Mass = moles x Mr.

Question 17

What is atom economy?

Answer 17

The percentage of reactants forming useful products. Also known as atom utilisation.

Question 18

What is the equation for atom economy?

Answer 18

Atom economy = relative formula mass of desired products / relative formula mass of all reactants
X100

Question 19

Why is a higher atom economy better?

Answer 19

High atom economy is better for profits and the environment.
Reaction with low atom economy use up resources quickly, they also make lots of waste materials that have to be disposed of somehow. That tends to make these reactions unsustainable. Low atom economy reactions are not profitable raw products are expensive to buy and the waste products are expensive to dispose of.

Question 20

What reactions have the highest atom economy?

Answer 20

The ones that only have one product.

Question 21

What is the percentage yield?

Answer 21

The amount of product you get is known as the yield. The percentage yield compares what you think you should get to what you actually get.

Question 22

What is the equation for percentage yield?

Answer 22

Percentage yield = mass of product actually made / maximum theoretical mass of product
X 100

Question 23

Why can’t the percentage yield be 100%?

Answer 23

Some product or reactant gets lost to the surroundings.
Not all reactants make a product eg in reversible reactions. (The products can turn back into the reactants)
There might be side reactions.
You lose some product when you separate it from the reaction mixture. Eg through filtering a liquid to remove solid particles.