AMOUNT OF SUBSTANCE Flashcards
define the 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.
define relative atomic mass
Relative atomic mass is the average mass of one atom
compared to one twelfth of the mass of one atom of carbon-12
define relative molecular mass
Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12
equation used to calculate the moles for pure solids ,liquids and gases:
*state the units
Moles=Mass/Mr
unit of mass:grams
unit of moles=mol
equation used to calculate the moles for gases:
*state the units
PV=nRT unit of pressure:pa unit of volume: m3 unit of temperature:K (+273) R=8.31
equation used to calculate for solutions:
unit of concentration:moldm-3
unit of volume: dm3
What is the Avogadro’s constant?
6.022*10^23
what equation is used to calculate no of particles or atoms?
moles*Avogadro constant
what is the equation used for density?
*include units
density=mass/volume
density=g cm-3
define empirical formula
An empirical formula is the simplest ratio of atoms of each element in the compound.
what is the general method used to calculate the empirical formula?
Step 1 : Divide each mass (or % mass) by the atomic mass of the element
Step 2 : For each of the answers from step 1 divide by the smallest one of those
numbers.
Step 3: sometimes the numbers calculated in step 2 will need to be multiplied up to give whole numbers.
These whole numbers will be the empirical formula.
define molecular formula
A molecular formula is the actual number of atoms of each element in the compound.
What is the formula for percentage yield?
percentage yield=actual yield/theoretical yield*100
what is the formula of atom economy?
percentage atom economy= mass of useful product/mass of all products*100
what is atom economy?
Atom economy is a theoretical measure of the proportion of atoms from the reactants that form the desired product
Why do companies in the chemical industry try to use processes with high atom economies?
-Better for environment
∵ produce less waste
-More efficient use of raw materials
-More sustainable (use natural resources more slowly)
-Less expensive
-Spend less on separating desired product from waste products & less on treating waste
method to making a solution:
- Weigh the sample bottle containing the required mass of solid on a 2 dp balance
- Transfer to beaker and reweigh sample bottle
- Record the difference in mass
- Add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid.
- Sometimes the substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved.
- Pour solution into a 250cm3 graduated flask via a funnel.
- Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask.
- Make up to the mark with distilled water using a dropping pipette for last few drops.
- Invert flask several times to ensure uniform solution.
Making a standard solution:
Why do you re-weigh the boat?
it may contain traces of solid
Making a standard solution:
Why do you rinse the beaker & stirring rod with distilled water and add that to the flask?
Makes sure no solute clings to beaker/rod
What do titrations allow you to do?
Allow to find out exactly the volume of acid needed to neutralise a measured quantity of alkali (& vice versa)
what is the method for carrying out a titration?
- rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)
- pipette 25 cm3 of alkali into conical flask
- touch surface of alkali with pipette ( to ensure correct amount is added)
- adds acid solution from burette
- make sure the jet space in the burette is filled with acid
- add a few drops of indicator and refer to colour change at end point
- phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [use if NaOH is used]
- methyl orange [yellow (alkali) to red (acid): end point orange][use if HCl is used] •use a white tile underneath the flask to help observe the colour change
- add acid to alkali whilst swirling the mixture and add acid dropwise at end point •note burette reading before and after addition of acid
- repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other
What colour is methyl orange in acids?
red
What colour is methyl orange in alkalis?
yellow
What colour is phenolphthalein in acids?
colourless
What colour is phenolphthalein in alkalis?
pink
Explain why the water used for rinsing has no effect on the accuracy of the titre (1)
water does not react with the reagents or change the number of moles of acid added.
State why its important to fill the space below the tap in the burette with solution
If the jet space is not filled properly prior to commencing the titration it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading
State why rinsing the inside of the conical flask with distilled water during titration can improve the accuracy of the endpoint (1
Returns reagent on the sides of the flask to the reaction mixture (to ensure that all of the acid/alkali reacts)
whys a conical flask used instead of a beaker?
it is easier to swirl the
mixture in a conical flask without spilling the contents.
why does repeating a titration make the average titre more reliable?
helps detect the anonymous result & remove it when calculating a mean
what is the percentage yield?
Percentage yield gives a measure of the efficiency of a reaction route. It is a measure of the percentage of reactants that have been converted into the desired product.
