C4 Chemical Calculations Flashcards
Why some elements have same relative mass as each other
Although the number of protons in the nucleus is the same for all atoms of the same element, the number of neutrons is not
Why some relative atomic masses may not be a whole number
Due to existence of isotopes having different mass numbers
Unit for amount of substance
Mole
23
1 mole = 6.02 x 10
Why chemical equations must be balanced
So it follows the law of conservation of mass
The effect of a limiting reactant on the amount of product made
Limits how much product can be formed
Limiting reactant
The reactant that gets consumed first in a chemical reaction
Theoretical yield
The maximum possible mass of a product that can be made in a chemical reaction
Actual yield
The actual mass of a product made in a chemical reaction
Percentage yield
(Actual yield ➗ theoretical yield) x 100
Why actual yield is often lower than theoretical yield
Some of the reactants do not react to form the product
Why percentage yield can never be over 100%
Incomplete reactions, some of reactants don’t react to form the product
How to calculate formula mass
Add masses of all atoms
Atom economy
A measure of efficiency of a chemical reaction
How to calculate atom economy
total number of atoms in the desired product
_________________________________________________________
total number of atoms in all the reactants
Why using reactions with high atom economy is important
Uses fewer natural resources
Produce less waste
Better for environment
Why the sum of the formula masses of the reactants is the same as the sum of the formula masses of the product
All atoms making up the reactants are still in the products
How concentration of solution can be changed
Increasing amount of solute
2 ways to calculate concentration of solution
Mass of solute ➗ volume of solvent
Number of moles of solute ➗ volume of solvent
How to calculate a titre
Values added together divide number of readings that were taken
Use of pipette for titration
To accurately measure a fixed volume of liquid
How an indicator can be used to determine the end point
Helps us spot the equivalence point in an acid-base titration
How precise results are obtained in a titration
Relies on accurate volume measurement
How accuracy can be improved in a titration
Calibrate your electrode regularly
Use of an indicator in an acid-base titration
To signal the end of a titration
How the volume of gas would change when temperature was changed
Volume increases as temperature increases
Molecules of gas have more kinetic energy
Percentage of mass of an element in a compound
Find percentage mass of sodium in sodium carbonate
Na2CO3
Na = 23
Na2CO3 = 106
23 x 2
________ x 100 = 43%
106
Surface area to volume ratio =
Surface area ➗ volume
What happens to mass in a chemical reaction
Mass is always conserved
Why are there always the same number of atoms on each side of reaction equation
No atoms are destroyed or created
If mass reaction may seem to change what could be a cause to this
Because one of the reactants is a gas found in air and all the products are solids, liquids or aqueous
Why does mass increase if one of reactants are a gas
Before reaction, gas is floating in the air but not contained in reaction vessel so can’t account for its mass
When gas reacts to form part of the products, it becomes contained inside reaction vessel so total mass inside reaction vessel increases
Why might mass decrease in a chemical reaction
Before reaction all rectants are contained in reaction vessel
If vessel isn’t enclosed gas can escape from reaction vessel as it’s formed
It’s no longer contained in reaction vessel so you can’t account for its mass
Total mass of stuff inside reaction vessel decreases
Why might mass seem to decrease in chemical reaction
One of products is a gas and all the reactants are solids, liquids or aqueous
When does a chemical reaction stop
When one of reactants is used up
Any others are in excess
What is amount of product formed directly proportional to
Amount of limiting reactant
How much volume does 1 mole of any gas occupy at 20°C
24dm3
20°C = atm = at room temperature
Volume of gas =
Mass of gas
______________ x 24
Mr of gas
Mr =
Relative formula mass
r.t.p
Room temperature and pressure
What’s the volume of 319.5 g of chlorine at r.t.p?
319.5
________ x 24 = 108dm3
71 (Cl2)
In the question
What’s the volume of 319.5 g of chlorine at r.t.p?
Why is it mass of Cl2 instead of Cl
Because chlorine naturally exists as a diatomic molecule in its gaseous form
So need 2 chlorine atoms
Elements that naturally exist as diatomic molecules in their gaseous form
Bromine
Iodine
Nitrogen
Chlorine
Hydrogen
Oxygen
Fluorine
What’s the concentration in g/dm of a solution of sodium chloride where 30 g of sodium chloride is dissolved in O.2 dm of water?
30 ➗ 0.2 = 150g/dm3
What’s the concentration, in mol/am?, of a solution with 2 moles of salt in 500 cm?
Convert cm3 to dm3 by ➗1000
500 ➗ 1000 = 0.5dm3
2 ➗ 0.5 = 4mol/dm3
Titration
Experiments that let you find the volumes needed for 2 solutions to react together completely
What does 0% percentage yield mean
No reactants were converted into product so no products was made
What does 100% percentage yield mean
You got all ghe product you expected to get
What are 3 common reasons some produtpct or reactant gets lost along the way
Not all reactants react to make a product
There might be side reactions
You lose some product when you separate it from the reaction mixture
Why can yield never be 100% in reversible reactions
The products can turn back into reactants
How could there be side reactions
Reactants could react with gases in the air or impurities in reaction mixture so end up forming extra products other than the one syou want
Mole
23
Mass of a substance that contains 6.02 x 10
particles
