One process used in gold mines to recover gold from mined rock is as shown below

Question

One process used in gold mines to recover gold from mined rock is as shown below:

___Au + ___NaCN + ___O + ___HO ___NaAu(CN) + ___NaOH

a) Balance the reaction

b) A small test batch is run in 10 mL water. (10 mL of water is 10 g of water) the sample in the reaction contains .0178 g of gold. It is treated with .0075 g of NaCN. O is present in excess (it being in the air). What is the limiting reagent in test reaction? (show calculations)

c) What mass of NaAu(CN) would be produced by the reaction described in question b?

Explanation / Answer

a) 4Au + 8NaCN + 1O2 + 2H2O ----> 4NaAu(CN)2 + 4NaOH

b) H2O moles = mass / molar mass of H2O = 10 /18 = 0.556

gold moles = 0.0178/196.966 = 0.00009

NaCN moles = 0.0075/49 = 0.000153

as per reaction for 0.00009 moles Au the NaCN moles required = ( 8/4) Au moles = ( 8/4) x 0.00009 = 0.00018

but we had only 0.000153 moles NaCN , hence NaCN is limiting reagnet

c) NaAu(CN)2 moles = ( 4/8) NaCN moles = ( 4/8) x 0.000153 = 0.0000765

mass of NaAu(CN)2 = moles x molar mass = 0.0000765 x 272 = 0.02 g

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