C4 - Quantitative chemistry Flashcards
Why might the conservation of mass not apply in a non-closed system?
-gas may have reacted and added on mass (eg metal oxides)
-gas could escape as product
What is a mole?
Avogadro constant, or 6.02x10²³ particles of a substance
Explain what a limiting reactant is, and why it would be useful to know:
-the reactant that has been fully used up is the limiting reactant, while the other is in excess
-useful to know as you can ensure one reactant gets fully used up by having an excess of the other reactant
How do you calculate a concentration?
remember to use dm³
A molarity is just a concentration but with moles
What is a percentage yield? Give its formula:
the mass of product formed compared with the maximum theoretical yield expressed as a percentage
State 3 reasons why percentage yield isn’t always 100%:
-reaction may not go to completion because it is reversible
-some product may be lost when separated from reaction mixture
-some reactants may react in different ways compared to the intended reaction
What is atom economy? Give its formula:
the percentage of the amount of reactants that end up as the desired products
In other words, the atom utilisation
Describe how you would calculate the atom economy of a reaction:
-find formula mass of the reactants
-find formula mass of the desired product
-remember to multiply each substance’s formula mass by its coefficient
-divide them
You have to multiply by the coefficients because it represents how many molecules of a particular substance is being used/formed, it wouldn’t make sense if you just ignored it because then the equation wouldn’t be balanced and atoms would be created/destroyed.
Describe the general steps necessary to perform a titration calculation:
-write out the values that you know in a table format
-deduce other values with their relevant calculations
What is a titration calculation for?
if the volumes of 2 solutions that react completely (usually acid + alkali) are known and the concentration of 1 of them is known, the concentration of the other can be calculated
What volume does x amount of any gas take up at RTP? Why? Give the relevant equation:
1 mole occupies 24dm³ at RTP (room temperature and pressure)
-equal amounts in moles of gases occupy the same volume if under the same conditions of temperature and pressure
remember to convert to dm³ for the volume in the equation below
What is this?
a BURETTE, not a biuret
RP2 - Describe the steps necessary to perform a general titration:
-put the acid in the burette, and the alkali in the conical flask on a white tile
-add phenolphthalein indicator
-add 1cm³ of acid at a time, swirling each time
-keep adding until a permanent colour change of pink to colourless is seen
-record final reading of the acid from the burette to get the titre
-repeat until you have concordant titres (within 0.1cm³ of each other)
-take mean of concordant titres only (there may be more than 2 concordant titres)
Usually what happens first is a rough titration, where you work out roughly how much you need to add, and then perform an accurate titration where you add most of it all in one go, then do it much slower around the required volume.
RP2 - Why are burettes used for titrations?
-can add solution in small increments
-more precise and accurate than a measuring cylinder
RP2 - Why should you use a burette for measuring out the acid added to the alkali, and a pipette for measuring out the volume of alkali in the conical flask?
-burettes can measure variable volume accurately
-but pipettes can measure a fixed volume accurately
