Topic 2 - RAM, RMM moles

Question 1

MASS NUMBER

ATOMIC NUMBER

Answer 1

Number of protons added to the number of neutrons.​

Number of Protons (same as the number of electrons)​

Question 2

Isotopes

Answer 2

Lots of elements have isotopes.​

Isotopes are atoms of the same element with the same number of protons and electrons, but different numbers of neutrons. They have a different mass number but the same atomic number​

Question 3

Relative atomic mass (Ar)

Answer 3

The Relative atomic mass (Ar) of an element, shown on the periodic table, is actually an average value​
(taking into account the ‘relative abundance’ of the isotopes of that element, as found on Earth.)​
​
Eg about 75% of chlorine has 18 neutrons, and 25% has 20 neutrons – so the average is (75 x 18) + (25 x 20) = 18.5​
100​
As Chlorine always has 17 protons, it’s atomic mass is shown on the periodic table as 17+18.5 = 35.5​
​

Question 4

Relative Atomic Mass (RAM)

Answer 4

Relative Atomic Mass (RAM) is shown on the Periodic Table. It sometimes has the symbol Ar. Relative atomic Mass is the average mass of an atom of an element compared to 1/12 the mass of an atom of Carbon-12​

Question 5

Relative Molecular Mass (RMM)

Answer 5

Relative Molecular Mass (RMM) has to be calculated. It sometimes has the Symbol Mr. It is the sum of the masses of all of the atoms in a molecule.​

Question 6

Avogadro’s Number

Answer 6

A mole is 6.02 x 1023 units of something​

A mole of an element contains 6.02 x 1023 atoms of that element​

A mole of a compound contains 6.02 x 1023 molecules of that compound​

The atomic mass (Ar) or formula mass (Mr) of a substance measured in grams will contain one mole of atoms or molecules of that substance

Question 7

Moles

Answer 7

The standard is Carbon-12, which has 6.02 x 1023 atoms of carbon and weighs exactly 12g​
​
1 mole of Hydrogen has 6.02 x 1023 atoms of hydrogen and weighs 1g​
​
1 mole of water has 6.02 x 1023 molecules of water and weighs 18g ( H2O = 1 + 1 + 16)

Question 8

Using Moles

Answer 8

Since we can’t actually carry out experiments with single atoms or molecules, we use moles to count how many atoms or molecules are involved​
​
We can calculate how many moles of something we have using the equation:​
​
Moles = Mass​/ Ar or Mr​

Question 9

Calculating empirical formulae

Answer 9

A compound contains 52.2% carbon, 13% Hydrogen and 34.8% Oxygen by mass. Calculate it’s empirical formula.​

Step 1 – Divide each mass by Ar for that element​
52.5/12 = 4.375, 13/1 = 13, 34.8/16 = 2.175​
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Step 2 – Divide each number by the smallest one​
4.375/2.175 = 2.01, 13/2.175 = 5.98, 2.175/2.175 = 1​
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If you have NEARLY whole numbers, these can be rounded up/down

If you still don’t have nearly whole numbers after step 2, you need to continue to simplify the ratio​

What you do to one number, you must do to them all​
It helps to know your fractions as decimals:​
0.5 = ½ so multiply everything by 2​
0.3 = 1/3 so multiply everything by 3​
0.25 = ¼ so multiply everything by 4 etc

Question 10

concentration equation

Answer 10

concentration mole dm-3 = amount mole / volume dm3