CC9 - calculations involving masses

Question 1

what is relative formula mass (Mr)

Answer 1

all the relative atomic masses (Ar) of the atoms in a compound added together

if there is 2 of an element, add 2x

Question 2

what is an empirical formula

Answer 2

the simplest whole number ratio of the atoms in a compound

Question 3

how to calculate the empirical formula of a compound from the masses of the elements it contains

Answer 3

step 1: divide mass of element by Ar of element eg. 3.2g oxygen/16 = 0.2
step 2: divide that by the smallest number found to get ratio eg. 0.2/0.2=1
step 3: write out ratio in full eg. 1:2
step 4: substitute in the chemical symbols eg. carbon:oxygen=1:2 so CO2

Question 4

define molecular formula

Answer 4

the actual ratio of each atom in one molecule, not simplified

Question 5

how to find the empirical formula, from the molecular formula of a compound

Answer 5

• divide each number of each element by the largest number that goes into all of them
• eg. glucose - C6H12O6
  largest number that divides is 6
  so empirical (simplest ratio) = CH2O

Question 6

how to calculate molecular formula from empirical formula and relative formula mass

Answer 6

step 1: find mass of empirical formula eg. CH3 = 12+1+1+1= 15
step 2: divide Mr by empirical mass
eg. 30/15=2
step 3: Multiply each subscript in the empirical formula by the answer in step 2 eg. CH3 x 2 = C2H6

Question 7

Describe an experiment to determine the empirical formula for a compound.

Answer 7

• weight a crucible and lid
• add a mg ribbon and weigh again
• subtract the mass of empty crucible and lid from crucible lid and mg ribbon to find the mass of mg
• heat crucible and mg using bunsen burner until mg turns white
• cool crucible and weigh again
• mass of MgO is mass after heating subtract initial mass of crucible and lid
• mass of gained O2 is mass of MgO takeaway mass of Mg

Question 8

MgO crucible experiment accuracy

Answer 8

• put lid on crucible to prevent loss of magnesium oxide
• leave a little gap so that oxygen can enter

Question 9

how to calculate empirical formula from MgO crucible experiment

Answer 9

• you have masses of both Mg and O2
  you can calculate the empirical formula from this
• experimental results not always 100% accurate, ROUND

Question 10

what is the law of conservation of mass
(closed system)

Answer 10

• no atoms are destroyed or created in a chemical reaction
• therefore, the should be the same number (mass) of atoms on each side of a reaction equation

Question 11

law of conservation of mass in a non-enclosed system

Answer 11

mass increase: one of reactants may be a gas found in the air and may react to form part of the product

mass decrease: if one of the products is a gas, and the reaction vessel isn’t enclosed, it can escape into the air

Question 12

Calculate the concentration of a solution in g dm–3.

Answer 12

how many grams in 1dm-3
mass (g) / volume (dm3(1L)) = concentration (g dm-3)

Question 13

how to calculate number of moles

Answer 13

moles = mass in grams/ Mr (of compound) or Ar (of element)

Question 14

What is avogardos constant

Answer 14

6.02x10^23

Question 15

How to find the number of molecules/ atoms in certain mass of a substance

Answer 15

Number of moles x avogardos constant