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What is the abbreviation of relative formula mass?



How do you calculate Mr (relative formula mass)

You add all the relative atomic masses (Ar) in the molecular formula.


Where do you find the Ar (atomic mass) of an element?

It is the bigger number on the periodic table, but on some exam questions they give it to you.


What is the formula for calculating the percentage mass of an element in a compound?

Percentage mass of an element in a compound
.Ar× num of atoms of that element
_________________________ ×100
Mr of the compound


What is a mole?

One mole of any substance is an amount of that substance that contains an Avogadro number of particles (6.02 × 10^23).

One mole of any substance will have a mass in grams equal to the relative Ar or Mr of the element or compound.


Oxygen has an atomic mass of 16. How much would one mole of oxygen weigh?



Carbon dioxide has a relative formula mass of 44. How much would one mole of carbon dioxide weigh?



What is the formula to find the number of moles in a given mass?

Number of moles (n) = mass (m) in g / Mr


What mass of carbon is there in 4 moles of carbon dioxide?
Mr of carbon = 12

M = n × Mr
= 4 × 12

There are 48g of carbon in 4 moles of carbon dioxide


What is the conservation of mass?

In a chemical reaction, mass is always conserved. That means that the mass of the reactants is always equal to the mass of the products.


Why might the mass appear to change in a chemical reaction?

If the mass increases, it's probably because it has reacted with a gas found in the air, and all of its products are solid, liquid or aqueous, and can therefore be measured.

If the mass decreases, it's probably because one of the products is a gas, and therefore couldn't be weighed.


What do the big numbers in front of the chemical formulas tell you? E.g. 2HCL

They tell you how many moles of each substance takes part or is formed by the reaction.
E.g. If it says 2HCl there is 2 moles of HCl in the equation.


What do the small numbers after the symbol mean?

How many atoms of each element there are in a substance.