Amount of substance

Question 1

define relative atomic mass

Answer 1

the average mass of one atom compared to 1/12th of the mass of one atom of carbon-12

Question 2

define relative molecular mass

Answer 2

average mass of a molecule compared to 1/12th of the mass of one atom of carbon-12

Question 3

unit for volume in ideal gas equation

Answer 3

m cubed

Question 4

unit for volume in concentration?

Answer 4

dm cubed

Question 5

how do i convert K to degreed celsius

Answer 5

+ 273

Question 6

converting between units

Answer 6

cm cubed - dm cubed ( divide by 1000)
cm cubed - m cubed ( divide by 100,000)
dm cubed - m cubed (divide by 1000)

Question 7

how do you calculate number of particles / atoms?

Answer 7

moles x avagrdos constant

Question 8

define empirical formula

Answer 8

the simplest whole number ratio of atoms of each element in a compound

Question 9

define molecular formula

Answer 9

actual number of atoms of each element in the compound

Question 10

method for water of crystallisation

Answer 10

The water of crystallisation in calcium sulfate crystals can be
removed as water vapour by heating as shown in the following
equation.
CaSO4
.xH2O(s) → CaSO4
(s) + xH2O(g)
Method. *Weigh an empty clean dry crucible and lid . *Add 2g of hydrated calcium sulfate to the crucible and weigh
again
*Heat strongly with a Bunsen for a couple of minutes *Allow to cool *Weigh the crucible and contents again
*Heat crucible again and reweigh until you reach a constant mass (
do this to ensure reaction is complete).

Question 11

why is using a lid for the crucible important?

Answer 11

lid improves the accuracy of the
experiment as it prevents loss of solid
from the crucible but should be loose
fitting to allow gas to escape

Question 12

why should you not use large amounts of salt in the experiment?

Answer 12

Large amounts of hydrated calcium sulfate, such as 50g,
should not be used in this experiment as the decomposition is
likely to be incomplete.

Question 13

why cant you use a wet crucible?

Answer 13

The crucible needs to be dry otherwise a wet crucible would
give an inaccurate result. It would cause mass loss to be too
large as the water would be lost when heating

Question 14

why cant you use very small amounts of solid in the experiment

Answer 14

Small amounts of the solid, such as
0.100 g, should not be used in this
experiment as the percentage
uncertainties in weighing will be too
high.

Question 15

how to make a standard solution?

Answer 15

• Weigh the sample bottle containing the required mass of solid
  on a 2 dp balance
• Transfer to beaker and reweigh sample bottle
• Record the difference in mass
• Add 100cm3 of distilled water to the beaker. Use a glass rod to
  stir to help dissolve the solid.
  *Sometimes the substance may not dissolve well in cold water so
  the beaker and its contents could be heated gently until all the
  solid had dissolved.
• Pour solution into a 250cm3 graduated flask via a funnel. * Rinse beaker and funnel and add washings from the beaker
  and glass rod to the volumetric flask. * Make up to the mark with distilled water using a dropping
  pipette for last few drops. * Invert flask several times to ensure uniform s

Question 16

why is a volumetric pipette more accurate than a beaker

Answer 16

Using a volumetric pipette is more accurate
than a measuring cylinder because it has a
smaller uncertainty

Question 17

hazards and safety risks with making the solution

Answer 17

Irritant - dilute acid and alkalis- wear googles
Corrosive- stronger acids and alkalis wear goggles
Flammable – keep away from naked flames
Toxic – wear gloves- avoid skin contact- wash hands after use
Oxidising- Keep away from flammable / easily oxidised materials

Question 18

units of ideal gas equation

Answer 18

Unit of pressure (P):Pa
Unit of volume (V): m3 Unit of temp (T): K
n= moles
R = 8.31 JK–1mol–1

Question 19

errors of using a gas syringe

Answer 19

*gas escapes before bung inserted
*syringe sticks
* some gases like carbon dioxide or sulfur dioxide are
soluble in water so the true amount of gas is not
measured.

Question 20

equation for % yield

Answer 20

actual/theoretical x 100

Question 21

equation for % atom economy

Answer 21

mass of useful products / mass of all reactants x 100

Question 22

why do chemists want high % yield?

Answer 22

Chemists want a high
percentage yield as means there
has been an efficient conversion
of reactants to products

Question 23

why do chemists want high % atom economy?

Answer 23

Chemists want a high
percentage atom economy so
that the maximum mass of
reactants ends up in the desired
product (so minimising the
amount of by-product).

Question 24

How do you carry out a titration?

Answer 24

*rinse equipment (burette with acid, pipette with alkali, conical flask
with distilled water)
*pipette 25 cm3 of alkali into conical flask
*touch surface of alkali with pipette ( to ensure correct amount is
added)
*adds acid solution from burette
*make sure the jet space in the burette is filled with acid
*add a few drops of indicator and refer to colour change at end point
*phenolphthalein [pink (alkali) to colourless (acid): end point pink
colour just disappears] [use if NaOH is used]
*methyl orange [yellow (alkali) to red (acid): end point orange]
[use if HCl is used]
*use a white tile underneath the flask to help observe the colour
change
*add acid to alkali whilst swirling the mixture and add acid dropwise at
end point
*note burette reading before and after addition of acid
*repeats titration until at least 2 concordant results are obtained- two
readings within 0.1 of each other

Question 25

why is a conical flask more preferable than a beaker?

Answer 25

A conical flask is used in preference to a
beaker because it is easier to swirl the
mixture in a conical flask without spilling
the contents

Question 26

why can distilled water be used?

Answer 26

Distilled water can be added to the conical
flask during a titration to wash the sides of
the flask so that all the acid on the side is
washed into the reaction mixture to react
with the alkali.
It does not affect the titration reading as
water does not react with the reagents or
change the number of moles of acid
added.

Question 27

how can you decrease an apparatus uncertainties

Answer 27

To decrease the apparatus uncertainties
you can either decrease the sensitivity
uncertainty by using apparatus with a
greater resolution (finer scale divisions ) or
you can increase the size of the
measurement made

Question 28

reducing uncertainties equipment

Answer 28

Replacing measuring cylinders with pipettes or burettes which have
lower apparatus uncertainty will lower the % uncertainty.
To reduce the % uncertainty in a burette reading it is necessary to make
the titre a larger volume. This could be done by: increasing the volume
and concentration of the substance in the conical flask or by decreasing
the concentration of the substance in the burette.

Question 29

reducing in measuring mass

Answer 29

Using a balance that measures to more decimal places or using a larger mass will reduce the % uncertainty in weighing a solid. Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the
mass added.