3. Quantitative Chemistry Flashcards
Why do chemists need to know how to write and balance equations?
- know the quantity of products
- know the quantity and proportions of reactant they’ll need
- determine quantity of waste product
What happens to mass during a reaction?
It is conserved
What is relative formula mass?
The sum of the relative atomic masses of each atom in a compound
What can relative formula mass be shortened to?
Mr
Why is there always uncertainty when results are obtained?
Due to errors
How to avoid a measurement error when using a measuring cylinder?
- measure from bottom on meniscus
* measure from eye level
What does judging the meniscus from eye level prevent?
A parallax error
What has to be ensured when using a gas syringe so no gas escapes?
That it is sealed tightly
What is a zero error when using a gas syringe?
When the gas syringe has not been set back to zero before taking the measurement
How can a zero error be prevented?
By setting the gas syringe or top pan balance back to zero before taking a reading
What is the result of a zero error?
A measurement appears larger than it actually is
What can disruptions to measurements when using a top pan balance be due to?
- other people (e.g. leaning on table)
- wind/breeze
- contamination (e.g. already powder on balance)
How can contamination of a top pan balance be avoided?
By cleaning balance with a soft brush
What are some reasons for uncertainty when collecting measurements?
- product/reactant is impure
- losses have occurred
- sample is wet
How can a product or reactant being impure affect certainty of measurements?
Less than 100% of the recorded mass is what you think it is
How can losses occurring affect the certainty of measurements?
Products like gases can escape easily, and solid powders can be blown away
How can a sample being wet affect the certainty of measurements?
Water can contaminate solids, so they appear heavier
What does the range do?
Represents the distribution of results
How is uncertainty shown using the mean and range?
Uncertainty = mean +/- furthest away figure from mean
How is percentage uncertainty calculated?
Range / Mean x 100
How many atoms, molecules or ions are in 1 mole?
6.02x10 to the power of 23
If you have 1 mole of an element, how much will it weigh?
The relative atomic mass in grams
How many atoms are in 12g - its relative atomic mass - of carbon?
6.02x10 to the power of 23
How is mass calculated? (Mr and mol)
Mr x Mol
How is Mr calculated? (mass and mol)
Mass / Mol
How is mol calculated? (mass and mr)
Mass / Mr
If you wanted to calculate the mass of a neutron relative to the mass of a proton, how would you do it?
Divide the neutron mass number by the proton mass number (which happens to equal 1, by the way)
What is the coefficient in a chemical equation proportional to?
The number of moles
Why would knowing the molar (stoichiometric) ratio be useful for a chemist to know how much of a reactant he needs?
He can calculate the mass of reactant needed using mass = mr x mol
What are the 3 steps to balance an equation using moles?
- Divide mass by Mr to calculate number of moles of each substance
- Divide each number of moles by the smallest to determine the ratio
- Molar ratio = coefficients in equation
What is the limiting reactant?
When the number of moles of a reactant is less than the stoichiometric ratio
What does the limiting reactant limit?
How far the reaction progresses i.e. the reaction will not go to completion
Why may it not always be possible to obtain the calculated amount of product?
- reaction may not go to completion because it’s reversible
- some product lost when being separated from reaction mixture
- some reactants may react different to expected
What is the yield?
The amount of a product obtained
What the is percentage yield?
When the actual amount of product obtained is compared to the theoretic amount as a percentage
What is the theoretical maximum?
The amount calculated using mass = mr x mol and if the reaction went fully to completion
How to calculate percentage yield?
Actual amount / Theoretical yield x 100
What is atom economy a measure of?
The amount of starting materials that end up as useful products
Why should reactions have a high atom economy?
For sustainable development and economic reasons
How is atom economy calculated?
Mr of desired product / Sum of Mr of all reactants x 100
What happens in atom economy equations that doesn’t happen in any other?
The Mr and coefficients are multiplied together
When calculating what do you multiply the Mr and coefficients together?
Atom economy
What are the chemical units of volume?
dm3
What is 1 dm3 equal to in litres?
1 litre
What is 1 dm3 equal to in cm3?
1000 cm3
How do you convert cm3 to dm3?
Divide by 1000
How is concentration (g/dm3) calculated?
Quantity of solute / volume
How can the quantity of a solute be calculated?
Concentration x volume
What is the equation that links concentration, quantity of solute and volume?
Concentration (g/dm3) = quantity (g) / volume (dm3)
What happens to concentration if volume is doubled?
The concentration is halved
What is the most common measure of concentration?
mol/dm3
Why do chemists prefer to use mol/dm3 when calculating concentration?
So they can use the molar stoichiometry of balanced equations
How do you convert from mol/dm3 to g/dm3?
Multiply by Mr
How do you convert from g/dm3 to mol/dm3?
Divide by Mr
How is concentration calculated using moles?
Concentration = mol / volume
How is the number of moles calculated using concentration?
concentration x volume
n=cv
What is the symbol equation for calculating concentration in mol/dm3?
n=cv
When calculating concentration with moles, what MUST the volume be in?
dm3
Factors that should be considered when choosing a method make a product?
- atom economy
- percentage yield
- reaction rate
- reversibility
- energy cost
- cost of reactants
- whether undesirable products can be sold/used
What do acids do when they are part of a solution?
Ionise to release H+ ions
What happens to pH when there is a higher concentration of H+ ions?
pH is lowered (more acidic)
What happens to pH when there is a lower concentration of H+ ions?
pH is raised (less acidic)
Is HCl a strong or weak acid?
Strong
Why is HCl is strong acid?
Every molecule fully ionises
How many molecules in weak acid ionise?
Few of them - partially ionises
Is ethanoic acid a strong or weak acid?
Weak - pH 3
Is nitric acid a strong or weak acid?
Strong - pH 1
Is citric acid a strong or weak acid?
Weak - pH 3
Is carbonic acid a strong or weak acid?
Weak - pH 4
Is hydrochloric acid a strong or weak acid?
Strong - pH 1
Is sulphuric acid a strong or weak acid?
Strong - pH 1
What type of scale does the pH scale use?
Logarithmic
What was Avogadro’s law about the molar volume of a gas?
1 mol of any gas at room temperature and pressure occupies a volume of 24dm3
In Avogadro’s gas law, what is room temperature?
25*C
In Avogadro’s gas law, what is room pressure?
1 atm
How much volume does 1 mol of a gas at room temperature and pressure occupy?
24dm3
Equation to find the volume of a known gas at room temperature and pressure?
Volume of gas = (Mass of gas / Mr of gas) x 24
How much CO2 is formed when 30dm3 of oxygen reacts with carbon monoxide? (2CO + O2 -> 2CO2)
1 mol of O2 makes 2 mols of CO2, so 30dm3 produces 60dm3 of CO2
