Chemical calculations Flashcards
How do you find the atomic mass of an element
Larger number on periodic table (protons + neutrons)
What is the equation relating Moles, Mr and Mass?
of Moles = Mass (g)/Mr
How many moles are there in 48g of Magnesium
48/24
= 2 moles
Calculate the mass of 4 moles of NaCl
4 x (23 + 35.5)
=234g
What is a mole of a substance
6.02 x 10^23 atoms/particles of any substance
How many moles of atoms are there in H20 ?
3 Moles
H2 = 2 moles O = 1 mole
2+1 = 3 moles of atoms
Calculate the number of atoms in 56g of Calcium Oxide
Ar Ca =40 ArO =16
56/56
= 1 mole of CaO
=6.02 x 10^23 atoms
2 moles of atoms in CaO
multiply by 2 2 x (6.02 X 10^23)
=1.204 x 10^24
Does the Mr include subscripts (delete this card is doesnt make the sensde
No :(
What is a limiting reactant?
A reactant that is fully used up
What is an excess reactant
A reactant that isn’t used up and is left in excess
How many moles of Zinc Iodide would be produced if we used 0.5 moles of Zinc and 1 mole of Iodine?
Ar Zn = 65 Ar I2 =254
Zn + I2 ===> ZnI2
a) 0.5 moles, as Zinc is the limiting reactant
b) Rearrange mole equation for mass:
Mass = Moles x Mr
Mass (ZnI2) = 0.5 ( 65 + { 127 x 2})
=159.5g
What is the equation relating Concentration, Mass and Volume?
Concentration (g/dm^3) = Mass (g) / Volume (dm^3)
What is the equation relating Concentration, Moles and Volume?
Concentration (Mol/dm^3) = # of moles / Volume (dm^3)
What is a solute
A substance dissolved in a solvent
If a solution’s mass increases, while maintaining the same volume, what happens to its concentration>
It increases
If a solution’s volume increases, while maintaining its mass, what happens to its concentration?
It decreases
Calculate the mass of a chemical needed to dissolve in a volume of 0.4 dm^3 to give a final concentration of 600g/dm^3
Rearrange for mass:
Mass = Concentration x Volume
Mass = 0.4 x 600
=240g
How do you get from cm^3 to dm^3 ?
divide by 1000
What is the yield of an experiment?
The mass obtained from a reaction
Why is it difficult to achieve an 100% yield?
- Some of the product may be lost when being separated
from the mixture - Reversible reactions may not react to completion
Why can’t a yield over 100% be achieved?
- It would mean that we’ve created atoms, which is impossible
What is the formula for percentage yield?
- Yield = ( Mass of actual product made / Maximum theoretical mass of product ) x 100
What is the atom economy?
- A measure of starting materials that end up as useful products
How do you calculate the Atom economy of a reaction?
- RFM of the desired product (from the reaction) / Sum of
the RFM’s of all the reactants x 100
What are the benefits of atom economy?
- Minimises the production of unwanted elements,
saves money and increases sustabinability
Why can’t the atom economy be over 100%?
- It would mean we’re creating atoms, which is
impossible
Calculate the atom economy:
C6H12O6 —– 2C2H5OH + 2CO2
(glucose) (ethanol) (carbon dioxide)
Desired product : Ethanol
Ar O = 16 Ar C = 12 Ar H = 1
RFM of desired product (ethanol) =
(2 x (12x2)) + ( 1 x 5) + 16 + 1 )
= 92
RFM of all reactants =
( 12 x 6 ) + 12 + (16 x 6)
= 180
100 (92/180)
= 51.1%
Are large numbers used in atom economy?
- YES
What is Avogrado’s law relating gas volumes, moles and room temperature?
- One mole of any gas occupies a volume of 24dm^3 at
room temperature and pressure
Room temp: 20 degrees celsius
Room pressure : 1 atmosphere
What is the equation relating the volume of a gas, and moles?
Volume (dm^3) = # of moles x 24
Calculate the volume of 51g of ammonia gas ( NH3)
Assume that the gas is at room temp and pressure
[ Mr NH3 = 17 ]
- # Moles NH3 = 51/17= 3 Moles
3 x 24 = 72
=72dm^3
Calculate the volume of of Hydrogen gas produced from 325g of Zinc. Assume that the gas is at room temperature and pressure. [Ar Zinc = 65 ]
Zn + 2HCL —— ZnCl2 + H2
- # Moles Zn = 325/65= 5 molesVolume H2 = 5 x 24
= 120dm^3
