3 Quantitative chemistry

Question 1

what does Mr stand for?
how do u calculate?

Answer 1

relative formula mass - relative atomic masses for all atoms in molecular formula added together
eg. MgCl2 -> Mg=24 Cl=35.5 24+35.5+35.5= 95

Question 2

what does Ar stand for?
where do u find on periodic table?

Answer 2

• relative atomic mass
• top number on the periodic table

Question 3

percentage mass of an element in a compound equation

Answer 3

(Ar x no. of atoms of that element/mr of compound) x100

Question 4

what is avogadros constant?

Answer 4

6.02x10^23

Question 5

what is a mole

Answer 5

• an amount that contains an avogadros number of particles
• one mole of atoms or molecules = mass in grams equal to relative atomic mass or formula of the substance eg. Carbon has Ar of 12 so it weighs 12g

Question 6

why may it seem that there is a mass decrease in a chemical reaction?

Answer 6

• probably due to one product being a gas whilst the reactants are solids, liquids or aqueous
• before the reaction all reactants are contained in a vessel
• if vessel isn’t enclosed the gas can escape from the reaction vessel therefore is no longer contained so you can’t acount for its mass
• so total mass of substances inside the reaction vessel decreases

Question 6

*

moles =

Answer 6

mass/mr

Question 7

why may it seem that there is a mass increase in a chemical reaction?

Answer 7

• probably due to one of the reactants being a gas and all products solids, liquid or aqueous
• before a reaction, gas is in air and not contained in reaction vessel so can’t account for its mass
• the gas reacts to form part of the product becomes contained in the reaction vessel so total mass increases