Formulae,equations and amounts of substance

Question 1

What is empirical formula

Answer 1

The simplest ratio of elements in a molecule

e.g C10H20 = CH2

Question 2

what are the basic steps to calculating the empirical formula from experimental data

Answer 2

1) Set up table with atom,mass,mole and mole ratio as headings
2) Fill in the table using information we are given(percentage mass or mass) or can find out
3) Divide mass by Mr of atom/molecule
4) Divide all answers by smallest answer
5) If need be, multiply by common denominator to get whole number
6) Use ratio of atoms to write formula

Question 3

What is molecular formula

Answer 3

Number of actual atoms in the molecule (not simplified)

Question 4

finding the molecular formula from empirical formaula

Answer 4

1) Mr of molecule divided by Mr of empirical formula

2) Multiply empirical by answer

Question 5

calculating the empirical formula using combustion analysis

Answer 5

1) calculate the mass of substance by calculating percentage mass
2) Continue by using table to fill in other values as if calculating empirical formula normally

Question 6

What is the equation for calculating moles

Answer 6

Amount in moles= mass of substance in grams divided by relative atomic mass

Question 7

When do we use the term ‘molecular mass’

Answer 7

Use it when referring to molecules (compounds containing a covalent bond)

Question 8

When do we use ‘relative formula mass’

Answer 8

Use it when referring to both ionic and covalent compounds

Question 9

what is ‘molar mass’ and when can it be used

- state its units

Answer 9

molar mass is the mass per mole of any substance either ionic or molecular
units = grams per mol = gmol-1

Question 10

Calculating molecular formula from empirical formula

Answer 10

• calculate the formula mass of the empirical formula and then divide it by the given RFM of the compound
• Then multiply the digits given in the empirical formula by the previous answer

Remember: In some cases you may have to calculate the empirical formula first before carrying out the other steps.

Question 11

State the ideal gas equation and when do we use it

Answer 11

Pv=nRT
we use this equation to find out the number of moles of gases in a certain volume that aren’t at room temperature or pressure

Question 12

state the ideal gas equation and what each letter stands for and its units

Answer 12

Pv=nRT

P= pressure (pa-pascals)
v= volume (m3 - metre cubed)
n= number of moles
R= gas constant (8.314J mol-1 K-1)
T= temperature (K=kelvin)

Question 13

state how to convert between these SI units for the ‘Ideal gas equation’

1) Kpa —> Pa
2) cm3 —-> m3
3) dm3 —> m3
4) ‘C —> K

Answer 13

1) multiply by (10x10x10)
2) multiply by 0.000001 or divide by 1000000
3) divide by 1000 or multiply by 0.003
4) add 273

Question 14

When do we use the ideal gas equation

Answer 14

If given a gas in a certain volume that isn’t at room temperature or pressure and are trying to find the amount of a substance in moles.
- can also be sued with volatile liquids(liquids that are a gas at room temperature and pressure)