3.1.2 - Amount of substance Flashcards
Define Relative Atomic Mass
The average mass of one atom of an element relative to 1/12th the mass of one atom of C-12
What is the formula for Relative Formula Mass?
(A1 x M1) + (A2 x M2) / TOTAL ABUNDANCE
Ideal Gas Equation
PV = nRT
Conversion from oC to Kelvin
oC to Kelvin –> ADD 273
What is an empirical formula?
The simplest ratio of atoms of each element in a compound
What is a mole?
The mole is the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12
Describe the steps of creating a Volumetric Solution
Common method taken from mark schemes:
1. Weigh the sample bottle containing the solid on a (2 dp) balance.
2. Transfer solid to beaker and reweigh sample bottle.
3. Record the difference in mass.
4. Add distilled water and stir with a glass rod until all the solid has dissolved.
5. Transfer to a volumetric flask with washings.
6. Make up to the 250cm3 mark with distilled water.
7. Invert flask several time
Describe weighing by difference
A method to weigh materials accurately
Mass of substance transferred = mass of weighing dish and substance - mass of dish after substance has been transferred
Why do we use a graduated volumetric flask when making standard solution?
Such that the solution has a known volume
Give two ways to reduce uncertainties in measuring mass
- Using a more accurate balance or a larger mass
- Weighing by difference to calculate the actual mass of solid transferred
If solution A is titrated against solution B, what’s in the conical flask?
Solution A is in the conical flask. It’s being ‘‘titrated against’’ Solution B in the burette
What is usually in the conical flask in titration?
alkali
What does concordant mean?
within 0.10 cm^3 of each other
means the results are accurate and reproducible
What is the main safety precaution in titration?
Wear eye protection and gloves (acids and alkalis are corrosive)
Why do we need to make sure the jet space in the burette is filled?
If it isn’t filled, it will lead to errors if it fills during the titration, leading to a larger than expected titre reading.
Why do we use a conical flask over a regular beaker?
It is easier to swirl the mixture without spilling hte contents
Why do we only use a few drops of indicator?
If too much is added, it will affect the ttration results
Why can distilled water be used during a titration to wash the sides of the flask?
Water does not affect the titration as it doesn’t change the number of moles of each reactant
How many titrations should we carry out? (at least)
3
What are the colour changes for phenolphthalein and methyl orange?
phenolphthalein [pink (alkaline) to colourless (acid)]
methyl orange [yellow (alkali) to red (alkali)]
When should we use phenolphthalein over methyl orange? Vice versa?
Use Phenolphthalein if NaOH is used
Use Methyl Orange if HCl is used
How do we reduce uncertainties in titration?
- Replacing measuring cylinders with pipettes / burettes which have lower apparatus uncertainty
- Increasing the volume and concentration of the substance in the conical flask to increase the titre volume
Why is a white tile used during titrations?
You can more easily see the colour changes and end points
What is the empirical formula?
The simplest whole number ratio of atoms of each element in a compound
What is theoretical yield?
The mass of product that is calculated and should have formed in a reaction
This is calculated using masses of reactants and a balanced equation
How do you calculate percentage yield?
Percentage Yield = Actual Yield / Theoretical Yield x 100
What does the percentage yield tell you?
Tells you how wasteful a process is, as it determines how much of the product is lost during the process
What is atom economy?
A way determining how wasteful the actual reaction is, how many atoms make products that are not useful
Atom economy is a measure of the proportion of reactant atoms that become part of the desired product, rather than by-products
How do you calculate atom economy?
% atom economy = molecular mass of the desired product/ sum of molecular masses of all reactants x 100
Why is a higher atom economy more beneficial? (4)
- High atom economy is desirable for industrial processes
- They are better for the environment as they produce less waste
- More efficient use of raw material means high atom economy is sustainable
- Sometimes waste is expensive to dispose of safely so less waste can be cheaper / less money has to be spent separating useful product form waste products
How can we remove water of crystallisation in compounds?
by heating, they can be removed as water vapour
Devise a method to remove the water crystals from hydrated calcium sulfate crystals (6)
*Weigh an empty clean dry crucible and lid .
*Add 2g of hydrated calcium sulfate to the crucible and weigh again
*Heat strongly with a Bunsen for a couple of minutes
*Allow to cool
*Weigh the crucible and contents again
*Heat crucible again and reweigh until you reach a constant mass ( do this to ensure reaction is complete).
Why shouldn’t we use small amounts of solid in experiments?
Percentage uncertainties in weighing will be too high
Accuracy Questions for Water of Crystallisation Experiment:
- Why does the crucible need to be dry?
- Why do we use a lid?
- Why do we not used large amounts of hydrated calcium sulfate?
- Why don’t we use small amounts?
- if the lid were wet, mass loss would be inaccurate as water would be lost when heating from the crucible
- lid prevents the loss of solid from the crucible
- large amounts may mean decomposition is likely to be incomplete
- percentage uncertainty is too high
Draw and describe the apparatus we can use to measure the volume of a gas in an experiment
( google doc/print )
What are some errors involved in using a gas syringe?
-gas escapes before bung inserted
-syringe sticks
-some gases like CO2 are soluble in water so true amount isn’t measured
What equation is used when changing the conditions of a gas?
p1v1 / t1 = p2v2 / t2
How can we calculate volumes of gases reacting?
-equal volumes of under gases under same conditions will have equal moles
-therefore, ratio of volumes = ratio of moles
What are the steps in working out what reactant is in excess
Step 1 - calculate amount of each reactant in mols
Step 2- use balanced equation to work out which is in excess
Step 3 - use the limiting reactant for calculations
How do you calculate % uncertainty?
(uncertainty / measurement made) x 100
Describe how we can carry out a titration. [7]
-Unknown [ ] Solution into conical flask using a pipette
-Known [ ] solution in burette
-Add 2 drops of indicator
-Run known solution into flask (slowly near endpoint)
-until indicator just changes colour
-Read volume from burette
-Repeat until concordant
