Opener – do on a sheet of paper (or GOOS Paper Opener – do on a sheet of paper (or GOOS Paper!) and we will be correcting in 5 minutes Calculate the percent composition of phosphoric acid, H3PO4 (used to give carbonated beverages a tart taste)

Empirical Formulas

Methanal, CH2O Ethanoic acid, C2H4O2 Same % composition! 40.0% C, 6.7% H, 53.3% O

Very different properties Methanal – formaldehyde and preservation of specimens

Ethanoic acid - vinegar

Therefore, the best way to identify an unknown is to identify its chemical formula. Let’s use the previous percent composition as our next example

40.0% C, 6.7% H, 53.3% O We want to work with a convenient sample size, so we will use 100 g We can do this because the percentages will be the same, no matter what our sample size is.

So now we are looking at 40.0 g of carbon, 6.7 g hydrogen, 53.3 g oxygen

Next we determine the amount

Next we determine the amount

CH2O What do we notice? The ratio of carbon : oxygen is just about 1:1 The amount of hydrogen is almost double this C:H:O = 1:2:1 CH2O

One way to determine ratios Divide each amount by the smallest amount. In this way the smallest amount becomes “1” and other amounts have higher values, relative to 1.

Empirical formula: Simplest whole number ratio of atoms or ions in a compound.

Molecular formula: Exact number of each type of atom in a compound.

Determining empirical formulas

example Determine the empirical formula of a compound with 35.4% Na, and the remainder is N.

What about more complex compounds?

69.9% iron, 30.1% oxygen

Ratios from fractions Whole number Multiply all subscripts by 0.25 = ¼ 4 (0.25) = 1 4 0.33 = 1/3 3(0.33) = 1 3 0.50 = ½ 2(0.5) = 1 2 0.67 = 2/3 3 (0.67) = 2 0.75 = ¾ 4 (0.75) = 3

Last One! 3 elements: 52.2% C, 6.15% H, 41.7% O