C3-4. Moles And Calculations Flashcards
What is the definition of an acid?
- An acid is a substance which dissociates in solution to release H+ ions.
- Therefore they are also referred to as proton donors.
What is the definition of alkali and base?
- Bases are substances which can accept H+ ions from other molecules.
- Alkalis are soluble bases.
What is a neutralisation reaction?
This is a reaction between H+ and OH- ions to form water.
What determines strong and weak acids?
Strong acids fully dissociate in solution to release H+ ions, whereas weak acids only partially dissociate in a reversible reaction.
What are polyprotic acids?
These are acids such as H2SO4 which contain more than one hydrogen atom. One H+ ion is released, and the resulting 1- ion partially dissociates.
What is percentage yield defined as?
The amount of product we produce during a reaction as a percentage of the amount we expect to form.
What factors affect percentage yield?
- Incomplete reactions
- Some product may remain within the reaction vessel
- Unexpected alternate reactions involving our reactants may have also occurred
- Some product may be lost during purification
Define atom economy.
- Atom economy is a measure of how much of the total product formed during a reaction is considered useful.
- Note that we do consider the coefficients of substances involved here.
What is the difference between empirical and molecular formulae?
- Molecular formulae provide the actual number of atoms present in a substance.
- Empirical formulae represent the simplest ratio of atoms present in a substance.
How can we work out empirical formulae from reaction data?
Generally, you should calculate the number of moles of ‘items’ involved in the formula of the unknown compound:
- C and H for hydrocarbons
- The salt and water of crystallisation for hydrated salts.
This allows you to calculated ratios of atoms in a given compound.
What calculations are generally used to process titration data?
- Generally, we need to work towards determining the number of moles present in the reacting solution, and the using that to determine concentration, mass etc. of the unknown substance.
- Remember that we may need to scale the number of moles, e.g. from 25cm3 in a burette to 250cm3 in a flask.
What is molar gas volume?
The volume a gas occupies at room temperature and pressure (RTP), generally given as 24dm3.
What is the ideal gas equation?
pV=nRT
- i.e. pressure * volume = amount * ideal gas constant * change in temperature
- With units given in Pa, dm3, mol, J mol-1 K-1, and K respectively.
What is the ideal gas constant?
8.314 J mol-1 K-1
What is a standard solution?
A solution of a known concentration, made by dissolving an exact mass of a solute in a solvent before making up the solution to an exact volume.
Why do we complete a rough titration?
To give us a general idea of the endpoint of the titration, so that we can be more accurate in future.
How do we record a mean titre, and why do we do this?
- We repeat the titration until two or more results fall within 0.05cm3 of one another, i.e. they are concordant.
- We then calculate an average from these values only.
- We do this to eliminate anomalous results from other attempts.
Why do we wash out the beaker and include the washings when we are making up a standard solution?
We do this to ensure that the entire measured amount of the solute we added to our solvent ends up in the solution we are going to make up.
Why do we weigh by difference when measuring the mass of the solid used to form a standard solution?
To allow us to calculate the true mass of solid added, accounting for any solid remaining in the weighing boat.
How can we ensure that we correctly identify when the endpoint of a titration is reached?
Place the conical flask on top of a white tile to make colour changes clearer. Use a suitable indicator with a noticeable colour change.
What is the definition of a hydrated salt?
- If a salt is hydrated, water molecules form part of the crystalline structure.
- We call this water the water of crystallisation, which is lost upon heating to form an anhydrous salt.
How do we determine the limiting reagent of a reaction?
Calculate the number of moles of each reactant, and compare this to the ratio of co-efficients in the chemical equation.
Why do we need to consider the limiting reagent in a reaction?
One of the reactants may be used up sooner than the other, meaning that this causes the reaction to stop whilst another reactant is left in excess, inhibiting the formation of other products - less may be produced than otherwise expected.
If a reaction only has one product, what can we say about its atom economy?
Atom economy must be 100%, as only the desired product is produced, with no by-products.