Calculations Involving Moles

Question 1

what do you need for the empirical formula

Answer 1

• how many grams are being used of each element
• total mole of that element
• division of grams by molecular mass
• ratio of 2 elements
• present simplified ratio and write it

Question 2

difference between molecular and empirical mass

Answer 2

• molecular mass is the sum of the atomic masses of all the elements
• empirical mass is the sum of the lowest ratio of the elements

Question 3

law of conservation of mass

Answer 3

• mass cant be created or destroyed only transferred

- mass of product must equal mass of reactants

Question 4

law of conservation of mass in a closed system

Answer 4

mass of the reactants has to be equal to the mass of the product

Question 5

what unit is concentration measured in

Answer 5

mol dm^-3

Question 6

what is 1000cm^3 in the unit of concentration

Answer 6

1dm^3

Question 7

concentrated solution and dilute solution in unit of concentration

Answer 7

concentrated = 200g/dm^3
dilute = 5g/dm^3

Question 8

what is a mole of a particle

Answer 8

the number of particles in a mole (1 gram) is avogadros number. 6.02x10^23

Question 9

what is the mass of a product fromed in a reaction controlled by

Answer 9

mass of reactant that is not in excess

Question 10

how do you calculate the moles of substances

Answer 10

• work out the sum of the moles (mass number)
• divide by the number of grams of substance being used
  90g of H2O, H+H+0= 18 so 90/18=5 moles
• moles = mass / relative formula mass

Question 11

how do you calulate the mass of substances

Answer 11

mass = moles * relative formula mass

Question 12

how do you calculate relative formula mass

Answer 12

relative formula mass = mass / moles

Question 13

what is the sum of the atomic mass actually called

Answer 13

relative formula mass

Question 14

when given a question that gives you a formula with the grams of a substance used and you are meant to work out the proportional mass of the other substance used, what do you do

Answer 14

• work out relative formula mass of substance with given mass
• work out its mass by the number of moles its multiplied by
• do the same for the other substance
• put values in a table with normal mass and used mass under it
• work out the multiplication or division used to get from the first substances normal mass to the used
• use that multi or divi to proportionalise the other substance
• if asked, add them up to have the product mass

Question 15

three reasons why the yield may be less than 100% in a reaction

Answer 15

• evaporation
• filtering
• transferring liquids

Question 16

why cant a compound like sodium chloride not have a molecular compound

Answer 16

because ionic compounds cant exist as molecular compounds

Question 17

what is the formula for concentration

Answer 17

concentration in g/dm3 = moles / volume

Question 18

how to work out the mass used in a concentration

Answer 18

mass = concentration x volume

Question 19

50cm^3 of potassium hydroxide solution of concentration 40g/dm3 is needed for an experiment. calculate the mass of potassium hydroxide that must be dissolved in water to make 50cm3 of colution of concentration

Answer 19

concentration = mass / volume
mass = con x volume 
mass = 40 x 50 (2000)
1dm3 = 1000cm3 so (2000 / 1000)
2g

Question 20

how do you work out molar volume

Answer 20

molar volume = gas volume / number or moles

Question 21

what is the equation to work out the volume of a substance in dm3 using the number 24

Answer 21

volume in dm3 = moles x 24

Question 22

when does the Formula for calcuating volume with 24 only work

Answer 22

when the room temperature and pressure is 20C and 1 atmosphere (RTP)