EL - Amount of Substance

Question 1

define mole:

Answer 1

amount of substance in grams that has the same number of particles as there are atoms in 12 grams in the carbon 12 atom

Question 2

what is the molar mass?

Answer 2

mass (g) in 1 mole of substance in gmol-1

Question 3

what is the molar gas volume?

Answer 3

volume of 1 mole of a gas at a given temp and pressure

at RTP the value is 24dm3mol-1

Question 4

what is avogadros constant?

Answer 4

6.02 x 10^23 atoms in 12 grams of 12-carbon

Question 5

what is the empirical formulae?

Answer 5

simplest ratio of atoms of each element in the compound

Question 6

how do you calculate the empirical formulae?

Answer 6

1. divide each mass by atomic mass of element (mr)
2. divide each number by smallest number
3. if not whole number = .3 x 3 / .5 x 2 etc

Question 7

what is a hydrated salt?

Answer 7

a salt that contains water of crystalisation
e.g Na2SO4 * 6H2O
hydrated copperr (III) nitrate (V)

Question 8

how do you calculate X in a hydrated salt?

Question 9

how do you remove crystals from an element?

Answer 9

via heating in a crucible to remove water as vapour

Question 10

what is the process to heating in a cucible?

Answer 10

1. weigh empty and dry (otherwise inaccurate results) crucible and lid
2. add 2g of (hydrated salt) to the crucible and weigh again with lid
3. heat witgh bunsen for couple of mins and allow to cool
4. weigh crucible and conents again
5. heat agin and reweigh until a constant mass (ensures reaction is complete)

Question 11

how do you calculate concentration of a solution?

Answer 11

n=cv

cm3 to dm3 = divide 1000
cm3 to m3 = divide 1000000
dm3 to m3 = divide 1000

Question 12

how do you convert dm3 into gdm3?

Answer 12

conc. in mol dm3 x mr

Question 13

how do you dilute a solution?

Answer 13

1. pipette 25cm3 of original solution into 250cm3 volumetric flask
2. make up to mark with distilled water - use dropping pipette for the last few drops until bottom of miniscus reaches level
3. invert flask several times to ensure uniform solution

volumetric flask is more accurate as smaller uncertaincy

Question 14

how do you calculate the conc of new dilution?

Answer 14

new dilute conc = original conc x (original vol / new vol)

Question 15

what is the ideal gas equation?

Answer 15

PV = nRT

p = pressure (Pa)
v = volume (m3)
n = moles
r = 8.314
t = temperature (k)

Question 16

what are the potential errors when using a gas syringe?

draw diagram of gas syringe

Answer 16

• gas escapes before bung inserted
• some gases like CO2 and S are soluble in H2O so the true amount of gas is not measured