SC9 - Calculations Involving Masses ✓

Question 1

SC9a - What is the difference between emirical and molecular forumla?

Answer 1

• Molecular formula is the actual amount of atoms of each element in a compound
• Empirical formula is the simplest whole number ratio of atoms of each element in a compound

Question 2

SC9a - How do you work out the empirical formul of a compound from the masses of each element?

Answer 2

• Find the moles by doing mass/Mr
• Divide both of the molar values by the smalles of the two
• e.g: 10g of Ca and 17.8g of Cl 10/40 = 0.25 17.8/35.5 = 0.5 0.25/0.25 = 1 0.5/0.25 = 2 1:2 ratio

Question 3

SC9a - Describe how you can find the empirical formula of magnesium oxide by heating magnesium ribbon.

Answer 3

• Place a tripod over a bunsen burner on a heat resistant mat and place a pipeclay triangle on the tripod
• Sand down the magnesium ribbon and measure its mass
• Meausre the mass of the crucible and lid
• Measure the mass of the crucible with the magnesium in and take away your previous result to get the starting value of magnesium
• Let the magnesium ribbon heat over the bunsen burner in the crucible, lifting the lid occasionally to let in oxygen
• Weight the mass of the crucible with the magnesium oxide and take away the mass of the crucible to find the mass of magnesium oxide
• Take away the mass of magnesium to find the mass of oxygne that reacted
• Use the emirical formula method to use these amsses to sork out empirical formula of MgO

Question 4

SC9c - What is the molar mass formula?

Answer 4

mol = mass/M_r or A_r (relative atomic or formula mass)

Question 5

SC9b - how do you work out the concentration of a solution?

Answer 5

Concentration (g dm-³) = mass of solute (g) ÷ volume of solution (dm³)

Question 6

SC9b - What is a dm³ equal to?

Answer 6

1L = 1000 cm³

Question 7

SC9b - What is the conservation of mass and what does it depend on?

Answer 7

• The idea that the mass of the reactants = the mass of the products in a closed system.
• In an open system, gases and liquids can sometimes escape altering this

Question 8

SC9b - How do you calculate the mass of a reactant needed to make a product (or vice versa)

Answer 8

• Work out the mols in the one you’ve been given the mass for and divide this by the big number next to it
• Multiply this by the big number next to the one you are trying to work out to give you the mols in this
• Multiply this by its Mr to get the mass
• Example: 2Al + 3Cl₂ → 2AlCl₃
• How many grams of Cl₂ to form 53.4g of AlCl₃
• AlCl₃ has an M_r of 133.5 so do 53.4 ÷ 133.5 = 0.4
• As the ratio of big number is 3:2 do 0.4 ÷ 2 and then x 3 to get 0.6 Do 0.6 x Cl₂s Ar whcih is 71 to get 42.6g

Question 9

SC9c - What is Avogadro’s constant?

Answer 9

The amount of particles in a mole 6.02x10²³

Question 10

SC9c - What is a limiting reactant?

Answer 10

• When you have the mass of two reactants, one is likely to be in excess meaning that once one is used up the one in excess will stiil have some remaining.
• As once one has been used up the reaction can’t continue, the one that gets used up first is the limiting reactant

Question 11

SC9c - How do you work out the limiting reactant?

Answer 11

• Firstly work out the balanced equation of the reaction (This may already be given to you)
• The big numbers show the ratio of mols needed
• Using the mass and M_r, work out the mols you have of one of them
• From here, work out the mols you need for the other one for the second one to not be the limiting reactant
• Then work out how many mols you have of the second one.
• If it is more than you would’ve needed then it is in excess if not then it is the limitjng reactant
• Use the values for the actual amount of mols used up to figure out the masses that will actually be used up

Question 12

SC9c - What is stochiometry?

Answer 12

The ratio of moles is a reaction