Mass, relative formula

For molecular and ionic substances, one mole of any substance is its relative molecular mass or relative formula mass expressed in grams. This is called the molar mass, i.e. the mass in grams of one mole of a substance. 

A given charge liberates different species in the ratio of their relative formula masses, divided by the number of electrons in the electrode reaction... 

A I he numbers of moles of the compounds in 100 g of water are obtained by dividing the mass of the compounds by their relative formula masses. The volumes of the solutions in dm arc obtained hy dividing the total mass bv the density. Dividing the numbers of moles of the halides by the volumes of their solutions gives the required molar concemralions. The answers are ... 

In the example above we can work out the relative formula mass of molecules such as 02 and C02 using the relative atomic masses of the atoms they are made from. The RFM is the sum of the relative atomic masses of all those elements shown in the formula of the substance. 

Next find the relative atomic mass of sodium from the Periodic Table (p. 136) and work out the relative formula masses of water, sodium hydroxide and hydrogen gas. 

Relative formula mass of hydrogen gas, H2, is 2 x 1 = 2 Now you can write these masses under the balanced chemical equation taking into account the numbers used to balance the equation. 

The mass of 1 mole of a compound is called its molar mass. If you write the molar mass of a compound without any units then it is the relative formula mass, often called the relative molecular mass (Mr). So the relative formula mass of water is 18. 

What is a the mass of 1 mole and b the relative formula mass (RFM) of ethanol, C2H5OH ... 

Element Compound Relative Formula Mass Weight for 1000 pg/l (PPM)-g/l Solvent Note... 

O relative formula mass the sum of ail the relative atomic masses of aS the atoms or ions in a compound... 

O mole the relative formula mass of a substance in grams... 

The relative formula mass of the iron oxide (Fe304) = ... 

Relative formula mass of citric acid (Aff of CftHg07) =. 

From the chemical equation for the reaction and using the relative formula masses together with the molar volume of a gas it is possible to predict the amounts of magnesium sulfate and hydrogen that arc produced when 24gof magnesium is reacted with excess sulfuric acid. 

The relative formula mass (RFM) equals the atomic or molecular weight of the substance, so for a compoimd such as sodium chloride (NaCl) this is simply the sum of the atomic weights of Na (23) and Cl (35.5), taken from the Periodic Table of the elements (see Appendix 2). The terms relative formula mass and relative atomic mass are often taken as interchangeable. 

RELATIVE ATOMIC MASS AND RELATIVE FORMULA MASS... 

In other words, the relative atomic mass of an element is the average mass of its atom to 1/12 of the mass of a atom. In the periodic table, the masses of atoms are written according to these relative calculations. For compounds similar to relative atomic mass, a relative formula mass is used. A relative formula mass is the sum of relative atomic masses of the atoms found in a compound. 

Calculate the relative formula mass of CO2 (Use periodic table in Appendix D)... 

Relative formula mass CO2 = 1 relative atomic mass C + 2 relative atomic mass O Relative formula mass C02= (1 12) + (2 16) = 44 amu... 

If the relative atomic masses of calcium, carbon and oxygen are 40, 12 and 16, respectively, what is the relative formula mass of calcium carbonate, CaCOs ... 

You need to be familiar with the use of exponential numbers in calculations and how to use your calculator. You should know how to calculate relative formula masses and be familiar with isotopes and the meaning of isotopic composition. You should also know how to balance equations. 

What is the relative formula mass of the compound sodium... 

This tells us that 2 molecules of hydrogen chloride (in hydrochloric acid) will react with 1 formula unit of calcium carbonate (as it contains a metal it must be an ionic compound), but it does not tell us how much hydrogen chloride in grammes we must add to the calcium carbonate to ensure that all the calcium carbonate is used up. But if we use moles all is much clearer. The relative formula mass of calcium carbonate is 40 (from calcium) -1-12 (from carbon) -I- (3 X 16) (from oxygen) = 100. So 1 mole of calcium carbonate has a mass of 100 g. Then for every 100 g of calcium carbonate we are reacting we will need to use 2 x 36.5g(=73g) of hydrogen chloride. 

If the relative formula mass of Al Si O is A, then rearranging gives ... 

The relative molecular mass is also called the relative formula mass especially for ionic substances. 

We could carry out similar analyses for other examples, such as the formation of magnesium oxide or calcium fluoride, covered earlier in the chapter. The relative formula mass is based on the common formula for a substance, even if it is not molecular. So magnesium oxide, a lattice containing an extensive array of ions, is said to have a relative formula mass of 32, although it does not contain discrete Mg-O units (see Chapter 4). 

Relative atomic mass, relative formula mass and molar mass... 

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