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C4 Chemical Calculations (Reacting Masses (Law of Conservation of Mass…
C4 Chemical Calculations
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Reacting Masses
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- write down the mass of the known substance
- write down the ratio of the two substances using the BIG multiplier numbers from the balanced equation
- work out the Ar / Mr of both substances (ignoring BIG no.)
- work out the no. of moles of the known substance using mass ÷ Ar / Mr
- work out no. moles of second substance using the ratios
- calculate the mass of the second substance using Ar/Mr x no. of moles
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Yields
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Percentage Yield
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- calculate the max. theoretical yield of the product using reacting masses
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Atom Economy
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- calculate the Mr of the 'desired product' including any BIG multipliers
- work out the total sum of the reactant's Mr including any BIG multipliers
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Empirical Formula
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Elements |___|___| Mass / % |___|___| ...........Ar |___|___| ......Moles|___|___| .......Ratio|___|___|
- write masses under the elements
- write in the Ar of each element
- calculate the no. of moles of each element by mass ÷ Ar (or % divide by Ar)
- find the simplest ratio by ÷ all the no. of moles by the smallest number
- use this ratio for the SMALL numbers of the elements in the compound
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Titrations
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Required Practical
- use a volumetric pipette and pipette filler to measure a known volume of alkali into a conical flask - allow all the liquid to drain out of the pipette
- add a few drops of acid/base indicator to the alkali in the conical flask and place the flask on a white tile so the colour change is more obvious
- (rise burette with distilled water and then with some acid) fill the burette with acid
- record the initial reading on the burette
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- open the burette's tap to release a small amount of acid into the alkali in the flask then mix the solutions
- repeat step 5 until the indicator just changes colour - this is the endpoint
- record the final reading on the burette and work out the volume of acid that has run into the flask (known as a titre)
- repeat entire process at least 3 times and try to get a pair of concordant results (results that are within 0.1cm³ of each other)
- discard any anomalous or rough (first) titres and calculate the mean using these results or using the 2 concordant results
Calculations
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to calculate an unknown concentration when both volumes and one accurate concentration of the solutions are known
- fill in the information from the question and make sure to convert cm³ to dm³
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- use the BIG multipliers in the balanced equation for the ratio
- calculate the amount of solute dissolved of the substance which has a known concentration and volume using the concentration equation (conc x vol)
- using the ratio, work out the amount (no. moles) of the other sustance
- calculate the concentration of the unknown substance in mol/dm³ using the concentration equation (amount ÷ vol)
- if your answer need to be in g/dm³, convert moles to grams (mol x Ar/Mr)
round your final answer
so that the no. of significant figures is consistent with the data given to the lowest no. of s.f. in the question
Volume of Gases
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to calculate mass
- make sure the volume is in dm³ and work out the no. of moles of gas using the volume of gas equation (vol ÷ 24dm³)
for reactions with solids or solutions that produce a gas when the volume of gas is provided in question
- using the ratio in the balanced equation and the moles of gas, determine the no. of moles of the other substance
- work out the mass of the other substance by using the equation Moles x Ar/Mr