CHEM 6A Lecture 8: CHEM W3 Lec 10/18/17
3 Jun 2018
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From last lecture:
E.g. A chemical analysis shows a compound contains boron and hydrogen only. It was
determined to have 81.10% B by mass. What is the empirical formula?
â—Ź %H? 100.00% - 81.10% = 18.90% H
1. Find # mol of all elements in a 100.00g sample
B: 10.811g/mol 10.811g B = 1 mol B
In compound: 81.10gB * 1 mol B = 7.5016 mol B
10.811g B
18,90g H * 1 mol H = 18.751 mol H
1.008g H
2. Find mol ratio and convert to whole numbers
â—Ź B:H = 7.5016 : 18.751
7.5016 7.5016
= 1 : 2.500
= 2: 5
â—Ź B2H5 is the empirical formula (EF)
3. The molar mass was found to be 53.3g/mol. What is the molecular formula?
â—Ź []molar mass of B2H5 = 2(MMB) + 5(MMH)
= 2(10.811g/mol) + 5(1.0079g/mol)
= 26.6615 g/mol
â—Ź Units of B2H5 per molecule:
â—‹ 53.5g * mol B2H5 = 2.00 mol B2H5
Mol molecule 26.6615 g mol molecule
â—Ź Molecular formula (MF) = B4H10
â—Ź ;]
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Document Summary
A chemical analysis shows a compound contains boron and hydrogen only. It was determined to have 81. 10% b by mass. 100. 00% - 81. 10% = 18. 90% h: find # mol of all elements in a 100. 00g sample. In compound: 81. 10gb * 1 mol b = 7. 5016 mol b. 18,90g h * 1 mol h = 18. 751 mol h. 1. 008g h: find mol ratio and convert to whole numbers. B2h5 is the empirical formula (ef: the molar mass was found to be 53. 3g/mol. []molar mass of b2h5 = 2(mmb) + 5(mmh) 53. 5g * mol b2h5 = 2. 00 mol b2h5. Summary (ef & mf: % g, g mol, simplest mol ratio (ef, mf from molar mass of compound. Main group, transition metal, and inner transition metal. Columns = groups 1 to 18 (similar chemical properties) Li - indicator turns purple, forms bubbles. Na - purple, faster color change more metallic (closer to bottom left corner of periodic table)
