chemical calculations

Question 1

what does Mr stand for

Answer 1

relative formula mass

Question 2

what is the definition of Mr

Answer 2

the total of the relative atomic mass added together

Question 3

what does Ar stand for

Answer 3

relative atomic mass

Question 4

what is the definition of relative atomic mass

Answer 4

the average mass of an element

Question 5

what is a mole

Answer 5

the amount of substance in the formula mass of a substance in grams

Question 6

what is Avogadro’s constant

Answer 6

6.02x10 ^ 23

Question 7

what is the equation for the number of moles

Answer 7

mass(g) ÷ Ar or Mr

Question 8

what is the definition of limiting reactant

Answer 8

The reactant that gets used up first OR the reactant that is not in excess

Question 9

what is the definition of yield

Answer 9

how much of a product is made in a chemical reaction

Question 10

what is the equation of percentage yield

Answer 10

percentage =( actual mass ÷ theoretical) × 100

yield (%) of product mass

Question 11

5 reasons why only few chemical reactions have yields of 100%

Answer 11

• 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

Question 12

what is the definition about atom economy

Answer 12

it is a measure of the amount of starting materials that end up as useful products

Question 13

what is the equation for atom economy

Answer 13

atom = (Mr of desired product ÷ Mr of reactants)×100
economy
(%)

Question 14

what is the equation of concentration using mass

Answer 14

concentration(g/dm^3) = mass (g) ÷ volume (dm^3)

Question 15

how much is 1 dm^3 in cm^3

Answer 15

1 dm^3 = 1000cm^3

Question 16

what are the 3 steps to calculate an unknown concentration using mass

Answer 16

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

Question 17

if a substance is more concentrated would it contain more or less solute in the same volume compared to a less concentrated substance

Answer 17

more

Question 18

what are 2 ways that you can increase the concentration of a solution

Answer 18

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

Question 19

what is titration used for

Answer 19

to measure accurately what volume of acid and alkali react together completely

Question 20

what type of reaction is titration

Answer 20

neutralisation

Question 21

what is the end point

Answer 21

the point in a reaction between an acid and alkali where it is complete

Question 22

what are the steps of titration to find an unknown concentration

Answer 22

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.

Question 23

what safety precautions should be used when performing titration

Answer 23

make sure you wear eye protection
fill burette below eye level
wear gloves
use pipette filler

Question 24

what is the equation of concentration including moles

Answer 24

concentration = moles (mol) ÷ volume (dm^3)

Question 25

what is the equation for moles of gas

Answer 25

moles of = volume of ÷ 24 dm^3
gas            gas (dm^3)