CHEM
Ex_1AKey Fall2015.pdf

# Calculate expected amount of zno based on the amount

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Calculate expected amount of ZnO based on the amount of O 2 we start with: 127.0 g O 2 × 1 mol O 2 32 g O 2 × 2 mol ZnO 3 mol O 2 × 81.39 g ZnO 1 mol ZnO = 215.3 g ZnO Now find the % yield: % yield = actual yield theoretical yield × 100 = 165.8 g ZnO 215.3 g ZnO × 100 = 76.99% If you chose the ZnS as the LR, you would get 261.06 g for theoretical yield and then 63.51%.

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A8 © 2015 L.S. Brown (12 pts) 7. Silicon and hydrogen form a variety of different compounds called silanes with the general formula Si x H y . When these compounds are burned in excess oxygen, SiO 2 and H 2 O are the only products formed. A 16.22-g sample of a particular silane was burned in O 2 , and 31.67 g of SiO 2 and 12.65 g of H 2 O were formed. What was the empirical formula of the original silane compound? The important thing to recognize is that all of the Si ends up in the SiO 2 and all of the H ends up in the H 2 O. So we can use moles of SiO 2 to find moles of Si and we can use moles of H 2 O to find moles of H. 31.67 g SiO 2 × 1 mol SiO 2 60.09 g SiO 2 × 1 mol Si 1 mol SiO 2 = 0.5270 mol Si 12.65 g H 2 O × 1 mol H 2 O 18.016 g H 2 O × 2 mol H 1 mol H 2 O = 1.4043 mol H Those are moles of each element in the original silane sample, so they give us the ratio of the 2 elements. Divide both by the smaller one: Si: 0.5270 0.5270 = 1 H: 1.4043 0.5270 = 2.66 The 2.66 is NOT a whole number, so we are not done. Multiply both by 3 to get both to be whole numbers, giving us: Si 3 H 8 for the empirical formula.
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