Chemistry formulas

Question 1

What is the formula for solubility?

Answer 1

solubility = mass of solid (g) / mass of water removed (g) x 100

Question 2

What is the formula for the relative atomic mass of a substance? (Mr)

Answer 2

All of the relative atomic masses added together e.g. MgCl2:

24 + (35.5 x 2) = 95

Question 3

How do you find the number of moles?

Answer 3

Moles = mass (g) / Mr

Question 4

What is the formula for percentage yield?

Answer 4

percentage yield = actual yield / theoretical yield x 100

Question 5

What is the formula for percentage composition?

Answer 5

Atomic mass of element / Mr of total substance x 100

Question 6

How do you calculate the empirical formula?

Answer 6

For example, C = 88.8%, H = 11.1%
Atomic mass of carbon = 12
Atomic mass of hydrogen = 1

88.8 / 12 = 7.4
11.1 / 1 = 11.1
<>
7.4 / 7.4 = 1
11.1 / 7.4 = 1.5
«1:1.5 is not a whole number ratio, we must simplify»
2:3
C2H3

Question 7

How do you calculate the molecular formula from the empirical formula?

Answer 7

https://www.youtube.com/watch?v=JeSSucG-CVw

Question 8

What is the formula for the concentration of a solution?

Answer 8

Concentration (mol/dm3) = number of moles / volume

Question 9

How can you convert the number of moles in a GAS at room temperature into volume?

Answer 9

By using Avogadro’s law which states that one mole of any gas occupies 24 dm3, so:

Volume (dm3) = moles of gas x 24

Question 10

What is the formula for metals reacting with acid?

Answer 10

Acid + Metal –> Salt + Hydrogen

Question 11

What is the formula for metals reacting with water?

Answer 11

Metal + Water –> Metal hydroxide + Hydrogen

Question 12

What is the formula for energy transferred?

Answer 12

Q = m x c x change in T

Energy transferred = mass of liquid x SHC x change in temperature

Question 13

What is the formula for enthalpy change?

Answer 13

Enthalpy change = total energy to break bonds - total energy to create bonds

Question 14

What is the formula for the rate of reaction?

Answer 14

Amount of reactant used or amount of product formed / time

Question 15

How do you calculate the mass of an unknown substance from an experiment?

Answer 15

1) Find the number of moles in the KNOWN substance
2) Find the moles in the UNKNOWN substance (using the ratio)
3) Find the mass of the unknown substance using the number of moles and Mr

Example:
What mass of hydrogen is needed to react with 40g of copper oxide?
CuO + H2 –> Cu + H2O

Ratio is 1:1:1:1 (number in front of the compound/element is the ratio number)

1) moles in CuO: 40 / 80 = 0.5mol
2) moles in unknown substance (hydrogen): 0.5mol
3) mass of hydrogen: 0.5mol x 2 = 1g