Chemical Analysis of a Copper-Gold Alloy: Formula, Carat, and Unit Cell Properties, Study notes of Chemistry

Download Chemical Analysis of a Copper-Gold Alloy: Formula, Carat, and Unit Cell Properties and more Study notes Chemistry in PDF only on Docsity! CHEM1002 2012-N-4 November 2012 • The diagram below shows the structure of an alloy of copper and gold with a gold atom at each of the corners and a copper atom in the centre of each of the faces. The length of the side of the cubic unit cell is 0.36 nm. = Au = Cu What is the chemical formula of the alloy? Marks 6 There are 8 Au atoms and each is on a corner so contributes 1/8 to the unit cell: total number of Au atoms = 8 × 1/8 = 1 There are 6 Cu atoms and each is on a face so contributes 1/2 to the unit cell. total number of Cu atoms = 6 × 1/2 = 3 Answer: AuCu3 or Cu3Au Pure gold is 24 carat, whilst gold alloys consisting of 75 % gold by weight are termed 18 carat gold. What carat gold is this alloy? The molar mass of Cu3Au is: molar mass = (3 × 63.55 (Cu) + 196.97 (Au)) g mol-1 = 387.62 g mol-1 The percentage gold is therefore: percentage gold = 196.97 / 387.82 ×100% = 50% As 100% gold is 24 carat and 75% gold is 18 carat, this corresponds to 12 carat. Answer: 12 carat What is the volume (in cm3) of the unit cell? The length of the side of the unit cell is 0.36 nm. This corresponds to 0.36 × 10-9 m or 0.36 × 10-7 cm. As the unit cell is cubic, its volume, V, is given by: V = (0.36 × 10-7)3 cm3 = 4.7 × 10–23 cm3 Answer: 4.7 × 10–23 cm3 ANSWER CONTINUES ON THE NEXT PAGE