Calculations Involving Masses (1+2)

Question 1

Define

empirical formula.

Answer 1

the simplest possible whole number ratio of atoms in each element within that compound

Question 2

Define

molecular formula.

Answer 2

the actual number of atoms of each element within the compound

Question 3

What is the

law of conservation of mass?

Answer 3

during a chemical reaction, no atoms are destroyed and no atoms are created

this means that there are the same number and types of atoms on each side of a reaction equation

Question 4

How would you calculate the

empirical formula of a compound from masses?

(5 steps)

Answer 4

1. Find the RFM of all the elements in the compound.
2. Divide the mass of each element by its RFM to find the moles of that element.
3. Write these moles as a ratio.
4. Divide each number in the ratio by the smallest number.
5. The smallest whole number ratio is the empirical formula.

n = m/RFM

Question 5

How would you

use an experiment to find empirical formula?

(8 steps)

Answer 5

1. Get a crucible and heat it until it’s red hot.
2. Leave the crucible to cool, then weigh it with its lid.
3. Add some clean magnesium ribbon to the crucible.
4. Reweigh the crucible, lid and magnesium ribbon.
5. Heat the crucible containing the magnesium. Put the lid on to stop any bits of solid from escaping, but leave a gap to allow oxygen to enter the crucible.
6. Keep heating the crucible strongly for abut 10 minutes, or until the magnesium ribbon has turned white.
7. Allow the crucible to cool and reweigh the crucible with the lid and contents.
8. Use the results to calculate the empirical formula of Magnesium Oxide.

Question 6

What are

solutions?

Answer 6

mixtures that contain the solute dissolved in the solvent

Question 7

Define

concentration.

Answer 7

the amount of substance in a certain volume of a solution

Question 8

What equation allows you to calculate

concentration?

Answer 8

concentration = mass of solute / volume

m = CV

m: g
C: g/dm^3
V: dm^3

Question 9

Define

limiting reactant.

Answer 9

the reactant that’s used up, causing the reaction to stop

this limits the amount of product that is formed

Question 10

Give the value of the

Avogadro constant.

Answer 10

6.02 x 10^23

Question 11

What is

one mole?

(of any substance)

Answer 11

the amount of that substance that contains 6.02 x 10^23 particles

(these particles could be atoms, molecules or ions)

Question 12

What equation links

RFM, moles and mass?

Answer 12

moles = mass / RFM

n = m / RFM

n: mol
m: g