chemical calculations Flashcards
what does Mr stand for
relative formula mass
what is the definition of Mr
the total of the relative atomic mass added together
what does Ar stand for
relative atomic mass
what is the definition of relative atomic mass
the average mass of an element
what is a mole
the amount of substance in the formula mass of a substance in grams
what is Avogadro’s constant
6.02x10 ^ 23
what is the equation for the number of moles
mass(g) ÷ Ar or Mr
what is the definition of limiting reactant
The reactant that gets used up first OR the reactant that is not in excess
what is the definition of yield
how much of a product is made in a chemical reaction
what is the equation of percentage yield
percentage =( actual mass ÷ theoretical) × 100
yield (%) of product mass
5 reasons why only few chemical reactions have yields of 100%
- the reaction may be reversible
- some reactants give unexpected/ unwanted products
- the reactants may not be pure
- some of the desired product may have been lost during its separation from the reaction mixture
what is the definition about atom economy
it is a measure of the amount of starting materials that end up as useful products
what is the equation for atom economy
atom = (Mr of desired product ÷ Mr of reactants)×100
economy
(%)
what is the equation of concentration using mass
concentration(g/dm^3) = mass (g) ÷ volume (dm^3)
how much is 1 dm^3 in cm^3
1 dm^3 = 1000cm^3
what are the 3 steps to calculate an unknown concentration using mass
1) calculate the mass in grams
2) calculate the volume in dm^3
3) use the equation concentration(g/dm^3) = mass (g) ÷ volume (dm^3) to get the unknown concentration
if a substance is more concentrated would it contain more or less solute in the same volume compared to a less concentrated substance
more
what are 2 ways that you can increase the concentration of a solution
1) by dissolving more solute in the same volume of its solution
2) evaporating some of the solution s you have the same mass of the solute in a smaller volume
what is titration used for
to measure accurately what volume of acid and alkali react together completely
what type of reaction is titration
neutralisation
what is the end point
the point in a reaction between an acid and alkali where it is complete
what are the steps of titration to find an unknown concentration
1) wash a pipette with distilled water and some of the alkali and put the known volume of the alkali in a conical flask using the pipette
2) add a few drops of Ph indicator inside
3) rinse a burette with distilled water and then with some acid then put the acid inside at a fixed volume e.g 2.5cm^3 and record it
4) put the conical flask under the tap and release the acid into the alkali slowly swirling it ensuring it is mixed in
5) repeat step 4 until the alkali has turned clear. Turn off the tap and measure the meniscus on the burette and note it down
6) repeat this process 3 times or until you have 2 results within 1cm^3 of (concordant) each other and calculate the mean.
7) use your results to work out the concentration of the alkali using moles.
what safety precautions should be used when performing titration
make sure you wear eye protection
fill burette below eye level
wear gloves
use pipette filler
what is the equation of concentration including moles
concentration = moles (mol) ÷ volume (dm^3)
what is the equation for moles of gas
moles of = volume of ÷ 24 dm^3 gas gas (dm^3)
