RELATIVE FORMULA or MOLECULAR MASS Doc Brown's Chemistry  GCSE/IGCSE/GCE (basic A level) O Level Online Chemical Calculations 2. Calculating relative formula mass or relative molecular mass RFM or M_{r} Quantitative Chemistry calculations online Help for problem solving in doing relative formula mass calculations using atomic masses. Practice revision questions on calculating relative molecular mass from a chemical formula and atomic masses. This page describes, and explains, with worked out examples, the method of how to calculate the relative formula mass of a compound (ionic or covalent) or the relative molecular mass of an element or a covalent compound. Online practice exam chemistry CALCULATIONS and solved problems for KS4 Science GCSE/IGCSE CHEMISTRY and basic starter chemical calculations for A level AS/A2/IB courses. These revision notes and practice questions on calculating the relative formula mass or calculating relative molecular mass calculations in chemistry and worked examples should prove useful for the new AQA, Edexcel and OCR GCSE (9–1) chemistry science courses. Spotted any careless error? EMAIL query ? comment or request a type of GCSE calculation not covered? 

2. How to calculate relative formula mass or relative molecular mass RFM/RMM or M_{r} How do I calculate relative molecular mass? RMM How to calculate relative formula mass? RFM Is there any difference between RMM and RFM? Does it matter whether the compound is ionic or covalent? The relative molecular mass/relative formula mass is defined as the sum of all the individual atomic masses of ALL the atoms in the formula (M_{r}). If the individual atomic masses of all the atoms in a formula are added together you have calculated the relative formula mass Atomic masses are listed at the bottom of the page e.g. for ionic compounds e.g. NaCl = 23 + 35.5 58.5) or molecular mass for covalent elements or compounds ... e.g. M_{r} of N_{2} = 28 from (2 x 14) or compounds e.g. M_{r} of C_{6}H_{12}O_{6} = 180 from [(6 x 12) + (12 x 1) + (6 x 16)], and more examples of how to calculate relative formula mass are further down the page, you get atomic masses from your periodic table. In a balanced chemical symbol equation, the total of relative formula masses of the reactants is equal to the total relative formula masses of the products (see law of conservation of mass calculations). To be honest, the term relative formula mass can be used with any compound whether it be ionic or covalent  it just seems NOT correct to talk about the molecular mass of an ionic compound when it doesn't consist of molecules, but is that one for the purists! The shorthand M_{r} can be used for the formula of any element or compound and to repeat, 'it doesn't matter whether a compound is ionic or covalent'.
Whereas relative atomic mass (section 1. Relative Atomic Mass) only applies to a single atom, anything with at least two atoms in the formula requires the term relative formula mass or relative molecular mass to be used. WARNING: The most common error is to use atomic/proton numbers instead of atomic masses, unfortunately, except for hydrogen, they are different! Examples of relative formula mass or relative molecular mass calculations: How to calculate relative molecular mass = How to calculate relative formula mass Molecular/formula mass = total of all the atomic masses of ALL the atoms in the molecule/compound. Watch out for brackets e.g. (OH)_{2} means two OH groups to add up!
Selfassessment Quizzes on relative formula mass or relative molecular mass type in answer QUIZ or multiple choice QUIZ
Above is typical periodic table used in GCSE sciencechemistry specifications in calculating relative formula mass or calculating relative molecular mass, and I've 'usually' used these values in my exemplar calculations to cover most syllabuses OTHER CALCULATION PAGES
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