Aim: How can we calculate percent composition of a compound? Do Now: 1.Take out a calculator and reference tables. 2.What is the gram formula mass of C 6 H 5 (OH) 3 ?
How can we find the molecular formula of a compound given the empirical formula and molecular mass? Example: If the empirical formula is CH, and the molecular mass is 26, what is the molecular formula? Three steps to solve: 1.Find the mass of the empirical formula. The mass of CH = 2.Divide the mass of the molecular formula by the mass of the empirical formula. Example: 3.Multiply the empirical formula by the answer found in step 2. Example:
Some More Practice Problems A compound has an empirical formula of CH 2 and molecular mass of 56. What is its molecular formula? The empirical formula is CH 3 and the formula mass is 30. What is the molecular formula?
What if we need to know the percent composition of a substance? The percent composition of a substance represents the composition as a percentage of each element compared with the total mass of the compound. The formula used can be found on reference table T. We can use this formula to find both the percent of individual elements in a formula or the percent of water in hydrated salt compounds (salts crystals that are attached to water).
Some Practice Problems Find the percent compositions of all the elements in the following compounds: 1.CuBr 2 2.NaOH
What if we are talking about a hydrated salt? HINT: Treat the water as a unit. Example: What is the percentage, by mass, of water in sodium carbonate crystals, Na 2 CO 3 10H 2 O?
The Lab Problem What is the percent, by mass, of water in the hydrated salt CuSO 4 5H 2 O?
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