% cobalt

A student finds absorbance = 0.261 for y max = 527 nm. She started with 0.1505 g of cobalt compound. What is her % cobalt?

Abs = [Cobalt] * E * 1

Molar absorptivity is 54 → From the table, the right side showed a value of 54 (closely)

[Cobalt] = 0.261 / 54 = 0.00483 M or 4.83 × 10^-3

How many moles of cobalt in 100 ml = 0.1 L?

ml to l =0.001 L

#mole cobalt = 0.00483 × 0.1 L = 4.83 × 10^-4

Mass of cobalt = moles * molar mass → 4.83 × 10^-4 * 58.93 = 2.846 × 10^-2 g or 0.02846

% cobalt = (2.2846 × 10^-2 g / 0.1505 g) * 100 = 18.91%

• repeat for 2nd trial

Get Average %, deviation, & 5 deviation

Average % Cobalt = 18.91 + 17.54 / 2 = 18.225%

Absolute deviation: | T1 % - Average % Co | + | T2 % - Average % Co | / 2 = %

So final result = Average Cl value +- Abs deviation → should look like 45.31 +- 0.33 %

% deviation: abs / average 100 → 0.33 / 45.31 * 100 = 0.73 %

→ meaning values are only 0.73 % miles away

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