C4 - Quantitative Chemistry

Question 1

Explain that mass is always conserved in chemical reactions

Answer 1

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

Question 2

Describe how to figure out the charge of an ion

Answer 2

The charge of ions can usually be figured out but the group of the element. For example:
-Ions in group 1 have a charge of +
-Ions in group 2 have a charge of 2+
-Ions in group 6 have a charge of 2-
-Ions in group 7 have a charge of -

Question 3

Determine the formula of an ionic compound using the charges on ions

Answer 3

In an ionic compound, the charges on the ions have to cancel out to leave an overall charge of 0.
If a non metal with a charge of 2+ reacts with a non metal that has a charge of 1-, it would mean the product would have a positive overall charge which would not be allowed, so there would have to be 2 non metal ions to cancel out the ions.

Question 4

State what is meant by relative formula mass

Answer 4

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 5

Calculate the percentage by mass of an element in a compound

Answer 5

The percentage by mass tells us what percentage of the relative formula mass is due to one of the elements in the compound.

Formula for percentage by mass:
( Total relative atomic mass of the atoms of that element / relative formula mass of the compound ) X 100.

Question 6

Calculate the number of moles of an element

Answer 6

Relative atomic mass of an element in grams is equal to 1 mole of that element.

Moles = mass (g) / relative atomic mass (Ar)

Question 7

State the formula to work out the moles of a compound

Answer 7

Moles of a compound = mass (g) / relative formula mass (Mr)

Question 8

Calculate the mass needed for a given number of moles

Answer 8

Mass = moles x relative formula mass

Question 9

Use moles to balance chemical equations

Answer 9

To balance chemical equations with moles, you find out the amount of moles in each reactant and place the number of moles before the reactant. Then you do the same with the product. To make sure the chemical equation is as balanced as it can be, you divide each number before the reactants and products by the smallest number of moles. If you get an answer of 1 mole, you don’t need to write the number before the reactant or product.

Question 10

Use Avogadro’s constant to calculate the number of molecules and atoms in a given sample

Answer 10

Avogadro’s constant is the number, 6.02x10^23
-If a molecule has a number of atoms then the mole of the molecules is the same as the number of atoms.
For example, 1 mole of methane molecules contains 5 moles of atoms.
-If you are asked to calculate the number of atoms in one molecule, you use the number 6.02x10^23.
For example, 1 mole of hydrogen chloride contains 2 moles of atoms so it contains (6.02x10^23)x2 atoms.

Question 11

Use moles to calculate the masses of reactants and products in chemical reactions

Answer 11

You find out the moles of the reactants and then use that number of moles to work out the mass of the product with the formula
Mass = moles X Mr

Question 12

Explain what is meant by a limiting reactant

Answer 12

In a chemical reaction, if you use the exact same amount of two reactants the product will leave behind no reactants. However, in most chemical reactions it is not practical to measure your reactants exactly, so one reactant has less than the other.
-This is done to make sure all of the reactant reacts.
-The reactant that is fully used up (the one with less) is known as the limiting factor.
-The reactant that is not fully used up (the one with more) is known as the excess reactant.

Question 13

Explain the effect of a limiting reactant on the amount of product in a reaction

Answer 13

The amount of moles that the limiting factor has is the amount of moles that the product will have.

Question 14

Calculate the concentration of a solution in g/dm^3

Answer 14

Concentration tells us the mass of a solute in a given volume of solution.
A solute is a chemical that is dissolved in a solvent.
Water is often used as a solvent.

-concentration = mass (g) / volume (dm^3)

Question 15

Explain how the mass of the solute and the volume of the solution affect the concentration

Answer 15

If the mass of the solute increases but the volume stays the same, then the concentration increases.

If the volume of the solution increases but the mass stays the same, then the concentration decreases.