topic 5: calculations

Question 1

What is RFM?

Answer 1

Relative formula mass (sum of all the Relative Atomic Masses of atoms in a compound)

Question 2

Are big numbers (coefficients) used in RFM?

Answer 2

No

Question 3

Moles =

Answer 3

Mass/RFM

Question 4

what are moles?

Answer 4

Moles are a way of measuring the amount of an element based on weight (e.g: 12g of carbon = 1 mole of carbon = 602000000000000000000000 atoms)

Question 5

Mass =

Answer 5

moles x RFM

Question 6

What is Avogadro’s constant?

Answer 6

6.02 x 10^23

Question 7

How do you calculate the number of atoms in a mole of a compound?

Answer 7

• Find the number of atoms in the compound (e.g: for H2O it would be 3)
• Multiply this number by Avogadro’s constant (6.02 x 10^23)

Question 8

How do you calculate the number of atoms in a given mass of an element?

Answer 8

• Find the number of moles
• Multiply this number by Avogadro’s constant (6.02 x 10^23)

Question 9

How do you calculate the number of atoms in a given mass of a compound?

Answer 9

• Find the number of moles
• Multiply this by the number of atoms in the compound
• Multiply this number by Avogadro’s constant (6.02 x 10^23)

Question 10

How do you balance an equation given masses and RFM of elements and/or compounds?

Answer 10

• Find all the moles
• Divide by the smallest number of moles to get the ratio
• Use these numbers as coefficients to the elements and/or compounds to balance

Question 11

How do you solve a reacting masses question?

Answer 11

• Find the ratio of the elements and/or compounds in the equation (the coefficients)
• Find the moles of the element or compound that you have been given the mass for
• Apply the ratio to this number (e.g: if the moles are 2 and the ratio is 1:2, you end up with 4 moles)
• Multiply this number of moles by the RFM to get the mass of the desired element/compound

Question 12

How do you solve a limiting reactant question?

Answer 12

• Find the ratio of the elements and/or compounds in the equation (the coefficients)
• Find the moles of all elements/compounds
• Apply the ratio to the moles
• Whichever element/compound that does not meet the ratio is the limiting reactant

Question 13

What is the unit of concentration?

Answer 13

g/dm^3

Question 14

concentration =

Answer 14

mass/volume

Question 15

How do you convert dm^3 to cm^3

Answer 15

1dm^3 = 1000cm^3