Quantitative Chemistry

Question 1

Percentage mass of an element in a compound

Answer 1

(relative atomic mass * number of atoms in that element/ relative formula mass of compound) *100

Question 1

What is the conservation of mass law?

Answer 1

During a chemical reaction, the total mass of reactants are = the total mas of products. No atoms are created or destroyed.

Question 2

If mass does change in a reaction in an unsealed reaction vessel what are the 2 main causes of this?

Answer 2

One of the reactants are a gas found in the air and all the reactants aren’t. One of the products is a gas and reactants aren’t.

Question 3

What does it mean if one of the reactants are gases found in the air and the products aren’t?

Answer 3

> Before the reaction, the gas is floating around in the air however, 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 in the reaction vessel - so the total mass of the contents inside the reaction vessel increases.

Question 4

What does it mean if the products are a gas and the products aren’t?

Answer 4

> 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 it’s mass is not accounted - the total mass oof the contents inside the reaction vessel decreases.

Question 5

What is a mole?

Answer 5

One mole of any substance is just an amount of that substance that contains an Avogadro’s number of particles - 6.02*10^23

Question 6

Number of moles equation

Answer 6

mass in g / relative formula mass

Question 7

How to balance equations using reacting masses steps…

Answer 7

1) Divide the mass of each substance by tits relative formula mass to find the number of moles.
2)Divide the number of moles by the smallest number of moles in the reaction to give the ratio of amounts of each substance in the reaction equation.
3)If any are still not whole numbers multiply by the same amount.

Question 8

Examples for changes in mass when reactants or products are gases.

Answer 8

When a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal or in thermal decompositions of metal carbonates carbon dioxide is produced and escapes into the
atmosphere leaving the metal oxide as the only solid product.