2 Empirical and molecular formulae

 Definition: An empirical formula is the simplest ratio of atoms of each element in the compound. Empirical formulae Step 1 : Divide each mass (or % mass) by the atomic mass of the element Step 2 : For each of the answers from step 1 divide by the smallest one of those numbers. Step 3: sometimes the numbers calculated in step 2 will need to be multiplied up to give whole numbers. These whole numbers will be the empirical formula. General method The same method can be used for the following types of data: 1. masses of each element in the compound 2. percentage mass of each element in the compound Example 7 : Calculate the empirical formula for a compound that contains 1.82g of K, 5.93g of I and 2.24g of O Step1: Divide each mass by the atomic mass of the element to give moles K = 1.82 / 39.1 I = 5.93/126.9 O = 2.24/16 = 0.0465 mol = 0.0467mol = 0.14mol Step 2 For each of the answers from step 1 divide by the smallest one of those numbers. K = 0.0465/0.0465 I = 0.0467/0.0465 O = 0.14 / 0.0465 =1 = 1 = 3 Empirical formula =KIO3Molecular formula from empirical formula Definition: A molecular formula is the actual number of atoms of each element in the compound. From the relative molecular mass (Mr) work out how many times the mass of the empirical formula fits into the Mr. Example 8 : work out the molecular formula for the compound with an empirical formula of C3H6O and a Mr of 116 C3H6O has a mass of 58 The empirical formula fits twice into Mr of 116 So molecular formula is C6H12O2 Remember the Mr of a substance can be found out from using a mass spectrometer. The molecular ion ( the peak with highest m/z) will be equal to the Mr. The Mr does not need to be exact to turn an empirical formula into the molecular formula because the molecular formula will be a whole number multiple of the empirical formula m/z