Measurement, Mass & Equations

The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.

True. The total mass of reactants is always equal to the total mass of products.

When a reaction mixture is open to the surroundings and a gaseous product is formed, the total mass will decrease as the gas escapes.

False. Mass cannot be created during a chemical reaction. The Law of Conservation of Mass states that mass is conserved.

Mr is the symbol for the relative formula mass and it refers to the total mass of the substance.

The mass of a reaction flask might increase if one of the reactants is a gas found in the air and all of the products are either solids or liquids.

False. The Law of Conservation of Mass applies to all chemical reactions, not just reactions in solution.

Molar mass refers to the mass of one mole of a substance.

The overall mass of the reactants and products remains constant / the same. Atoms are rearranged as reactants form new chemicals / products.

To calculate the relative formula mass of a substance, you have to add up the relative atomic masses of all the atoms present in the formula.

False. Relative molecular mass does not apply to ionic compounds.

The relative formula mass of CuCO3 is: 63.5 + 12 + (16 x 3) = 123.5

The relative molecular mass of C5H5N is: (12 x 5) + (1 x 5) + 14 = 79

The difference between relative molecular mass and relative formula mass is that relative moleculas mass only applies to covalent compounds. Relative formula mass applies to all compounds.

The relative molecular mass of NH4NO3 is: 14 + (1 x 4) + 14 + (16 x 3) = 80

The equation to calculate the percentage by mass of an element in a compound is: (Ar × number of atoms of the element) / (Mr of the compound) × 100

True. The relative formula mass of CaCO3 is = 40+12 +(16 x 3) = 100

True. The percentage composition of elements in a compound always adds up to 100%.

False. The percentage composition of oxygen in carbon dioxide, CO2, is: (32 / 44) x 100 = 72.7 % This is because CO2 contains 2 oxygen atoms.

The percentage by mass of hydrogen in water (H2O) is: (2 / 18) x 100 = 11.1 %.

False. Ar (Mg) = 24 Ar (O) = 16 Mr (MgO) = 40 So, the percentage by mass of Mg in MgO is 24 / 40 x 100 = 60%.

True. The mass of nitrogen in ammonium nitrate, NH4NO3, is 28 because there are two nitrogen atoms in the compound.

The percentage of fluorine in tin(II) fluoride, SnF2 is: Mass of fluorine = 2 x 19 = 38 Mass of tin(II) fluoride = 119 + (2 x 19) = 157 Percentage of fluorine = 38 / 157 x 100 = 24.2%.

True. The mass always increases when a metal reacts with oxygen.