Topic 3- Quantative Chemistry

Question 1

What is the relative formula mass?

Answer 1

• Total atomic mass id all the atoms in the molecular formula added up.

Question 2

Percentage mass of an element in a compound equation

Answer 2

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

Question 3

What is a mole?

Answer 3

• The name given to an amount of substance that contains an avogadro’s number of particles.

Question 4

Number of moles equation

Answer 4

Number of moles= mass (g)/Mr

Question 5

What does the conservation of mass state?

Answer 5

• In a chemical reaction, no atoms are destroyed and no stoms are created.
• This means there are the same numbee and types of atoms on each side of a reaction equation.
• Because of this, no mass is lost or gained.

Question 6

Why would the mass increase during a reaction?

Answer 6

• Most likely because one of the reactants is a gas thats found in air.
• Before the reaction, the gas is floating around in the aid. 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 substance inside the reaction vessel increases.

Question 7

Why does the mass decrease during a reaction?

Answer 7

• Most likely because 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 as it’s formed. It’s no longer contained in the reaction vessel, so you can’t account for its mass- the total mass of the susbtances inside the reaction vessel decreases.

Question 8

What is a limiting reactant?

Answer 8

• The reactant that gets completely used up in a reaction before the rest.

Question 9

Atom economy equation

Answer 9

Atom economy= relative formula mass of desired products/relative formula mass of all reactants x 100

Question 10

Why are yields always less than 100%?

Answer 10

• In reversible reactions, the products can turn back into reactants, so the yield will never be 100%.
• The reactants might react with gases in the air, or impurities in the reaction mixture, so they end up forming extra products.
• When a liquid is filtered to remove a solid, you nearly always lose a bit of liquid or a solid.