Relative Formula Mass (GCSE Chemistry)
Relative Formula Mass
Relative Formula Mass
Relative Formula mass (Mr) is the addition of all the relative atomic masses of atoms in the molecular formula It is important to look at how we can calculate it.
Practice Question: What is the relative formula mass of CaCO3?
1. Find the relative atomic masses. We need to know the relative atomic masses of each atom in the compound. To do this, we can use a periodic table.
Calcium = 40, Carbon = 12, Oxygen = 16
2. Check for multipliers and subscript numbers. In this instance, oxygen has a subscript 3 after it, indicating that there are three atoms.
This means that we need to multiply the Ar of oxygen by 3.
3 atoms of oxygen
16 x 3 = 48
3. Add up the total. To find the relative formula mass, we simply need to add up all our relative atomic masses.
40 + 12 + (16 x 3) = 100
Mr of CaCO3 = 100
Balanced Chemical Equations
Relative formula mass is linked to conservation of mass. We can use relative formula mass to see whether mass is conserved in a reaction
H2 + O → H2O
Relative formula mass should be equal in a balanced chemical equation. If a chemical equation is balanced, then relative formula mass will be equal on both sides. The Mr of the reactants will be equal to the Mr of the products.
Practice Question: Show that mass is conserved in this reaction: 2Na + Cl2 → 2NaCl.
1. Add up the Mr of the reactants. The first step is add up to the relative formula masses of all the reactants.
Mr(Na) = 2 x 23 = 46
Mr(Cl2) = 2 x 35.5 = 71
Overall Mr of the reactants = 46 + 71 = 117
2. Add up the Mr of the products. Next, add up the relative formula masses of the product:
Mr(NaCl) = 2 x (23 + 35.5) = 117
3. Check whether mass is construed. To do this, check whether the Mr of the reactants and of the products are equal.
Overall Mr of the reactants = 117
Overall Mr of the products = 117
Mass is conserved in this reaction.
Relative formula mass (also known as molecular weight or formula weight) is a measure of the mass of a molecule or a formula unit, relative to the mass of a carbon-12 atom. This unit is used in chemistry to compare the masses of different substances.
Relative formula mass is important in chemistry because it provides a way to compare the masses of different substances and to calculate the number of atoms or molecules in a sample. This information can be used to determine the amount of a substance needed for a reaction, as well as the expected outcome of the reaction.
Relative formula mass is calculated by adding up the atomic masses of all the atoms in a molecule or formula unit. The atomic masses of the elements are taken from the periodic table.
Relative formula mass and molar mass are similar concepts, but molar mass refers to the mass of one mole of a substance, while relative formula mass refers to the mass of a single molecule or formula unit. One mole of a substance contains Avogadro’s number of molecules or formula units.
Molar mass is related to relative formula mass because it is simply the mass of one mole of a substance, which is equivalent to Avogadro’s number of molecules or formula units of that substance. To calculate molar mass, you multiply the relative formula mass by Avogadro’s number.
The formula for calculating molar mass from relative formula mass is: Molar Mass = Relative Formula Mass * Avogadro’s number.
Relative formula mass cannot be used directly to determine the number of atoms in a sample, but it can be used to determine the number of molecules or formula units in a sample. To determine the number of atoms in a sample, you need to know the number of molecules or formula units and the formula of the substance.
The concept of relative formula mass is closely related to the mole concept, as it provides a way to compare the masses of different substances and to calculate the number of atoms, molecules, or formula units in a sample. The mole concept provides a way to quantify the amount of a substance in a chemical reaction.
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