Survey

# Download Finding Empirical and Molecular Formulas (1A)

Document related concepts

Transcript

Finding Empirical and Molecular Formulas 1. Convert all masses to moles of each element. If %-mass are given, assume 100 g of the sample (so that % = massing) and then convert to number of moles using the respective atomic masses. 2. Find which number of moles is the smallest and divide all the different numbers of moles by that smallest number. If the ratios you obtain are all whole numbers, use the numbers as subscripts to give the formula. This is the empirical formula! 3. If you do not get whole numbers, then multiply everything by a suitable number that removes the fraction. Common fractions If number ends in: multiply by: 0.25 (1/4) x4 0.333 (1/3) x3 0.5 (1/2) x2 0.666 (2/3) x3 0.75 (3/4) x4 4. To get the molecular formula, you also need the approximate Molar Mass (aprox MM) of the compound (this mass may be determined experimentally and it should be a piece of information in the problem) and the Molar Mass of the Empirical Formula (MMEF), which you can easily calculate. Get the ratio of both molar masses as: n aproxMM MMEF Multiply the Empirical Formula by this number n to get the Molecular Formula. You must round n to the closest integer. EXAMPLE: Q. A compound is 78.14% B and 21.86%H by mass. Its molar mass is approx. 30 g/mol. Determine the empirical and molecular formulas. 1. Assume 100g sample so that % goes directly to mass in grams and then convert to number of mol. 1molB 78.14 gB 7.2285molB 10.81gB 1molH 21.86 gH 21.687molH 1.008gH 2. Divide by smallest #mol (in this case the moles of B) 7.2285molB 21.687molH 3H and 3 1B 7.2285molB 1B 7.2285molB Since we obtained whole numbers for the ratios, these are the numbers to use for the empirical formula. Empirical Formula: BH3 3. (We skip this part of the procedure since we got whole numbers previously!) 4. aproxMM = 30 g/mol, MMEF 10.81 3(1.008) 13.834g/mol aproxMM 30 g / mol round to 2. 2.168 MMEF 13.834 g / mol So, the molecular formula is the empirical formula multiplied by 2: n Reyes Molecular Formula is B2H6.