C4 - chemical calculations

Question 1

what is the law of conservation of mass?

Answer 1

mass cannot be created or destroyed in chemical reactions.

Question 2

what is the relative atomic mass (Ar) of a substance?

Answer 2

The average mass of the atoms of an element compared with carbon-12.

Question 3

How do you calculate the relative atomic mass (Ar) of a substance?

Answer 3

isotope mass x isotope abundance / 100

Question 4

what is relative formula mass (Mr)?

Answer 4

the total of the relative atomic masses, added up in the ratio of the chemical formula of a substance.

Question 5

how do you calculate relative formula mass (Mr) ?

Answer 5

add up all the relative atomic masses of the atoms in a compound.

Question 6

what are moles?

Answer 6

the amount of a substance in the relative atomic or formula mass of a substance in grams.

Question 7

what is thermal decomposition?

Answer 7

breaking down of a substance into smaller molecules by heating.

Question 8

how do you work out percentage yield?

Answer 8

acual yield / theoretical yield x 100

Question 9

what are some reasons that reactions dont have 100 percent yield?

Answer 9

-some reactants may react to give side products
-some of the product may be left behind in apparatus
-reactants may have not been completely pure
-

Question 10

revise reacting masses calculation

Question 10

revise limiting reagnets

Question 10

what calculations should you do to work out concentration?

Answer 10

-calculate Mr of solute
-calculate moles of solute
-calculate vol of solvent in dm^3
-calculate concentratio of Solution in mol/dm^3

Question 11

what is a standard solution ?

Answer 11

a standard solution is a solution of accurately known concentration prepared from a primary standard, that is weighed accurately and made up to a fixed volume.

Question 12

what is a primary standard?

Answer 12

a compound which is stable, of high purity, highly soluble in water and a high moar mass to allow for accurate weighing

Question 13

what is the method for making a standard solution?

Answer 13

1) weigh the mass of the solute
2) dissolve solute in water
3) rinse ot with distilled water to make sre no solute left
4)add distilled water until at mark.

Question 14

what is the method for a titration

Answer 14

1) use a pipette to transfer 25cm³ of sodium hydroxide solution into a conical flask
2)Add 5 drops of indicator such as Phenolphthalein to the alkali in the conical flask
3)place the conical flask on a white tile so we can see a colour change more clearly
4)fill a burette with sulfuric acid
5)add acid to the alkali until the solution is neutral. we need to add the perfect amount for this to happen. once we start to see a colour change go drop by drop.
6)read the volume of acid added from the burette.

Question 15

what is meant by the end point?

Answer 15

when the indicator changes colour during a titration

Question 16

why would universal indicator not be suitable to use in a titration?

Answer 16

as you need a clear colour change from acid to alkali

Question 17

what are 3 things you do during a titration to ensre accurate results.

Answer 17

-all equipment washed with deionised water before use
-the burrete is read where the bottom of the meniscus is
-titration is repeated to give concordent results

Question 18

what is the volume of 1 mole of any gas at room temperature and pressre?

Answer 18

24dm^3 or 24,000 cm^3 ( this is called the molar volume of a gas )

Question 19

what is the volume of a gas at room temp?

Answer 19

number of moles x 24

Question 20

is damp gas collection or dry gas collection more accurate?

Answer 20

Always dry gas collection as there is less oppotunities for the gas to escape

Question 21

how do you work out atom economy?

Answer 21

mass of atoms in desired product / mass of atoms in reactant x 100

Question 22

what is emperical formula ?

Answer 22

the simplist ratio of whole numbers of atoms in a compond.

Question 23

what is the equation for Mr?

Answer 23

Mr = mass / moles

Question 24

what is the equation for concentration?

Answer 24

concentration = moles / volume

Question 25

what is the eqation for the volume of a gas?

Answer 25

volume = moles x 24

Question 26

What steps would you follow to calculate the mass in a reaction?

Answer 26

1) Write out the balanced equation
2) Find the moles of the known substance (moles = mass ÷ Mr)
3) Find the moles of the unknown substance (using the molar ratio from the balanced equation)
4) Find the mass of the unknown substance (mass = moles x Mr)