J.R. S. answered 03/28/17
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3(NH4)2PtCl6 ==> 3Pt(s) + 2NH4Cl + 2N2 + 16HCl balanced equation for decomposition
moles of Pt desired = 6.2 g x 1 mole/195 g = 0.032 moles
moles (NH4)2PtCl6 needed = 0.032 moles Pt x 3 moles (NH4)2PtCl6/3 mole Pt = 0.032 moles (NH4)2PtCl6 needed
mass (NH4)2PtCl6 needed = 0.032 moles x 444 g/mole = 14.2 g needed
At 85% yield, mass (NH4)2PtCl6 needed = 14.2 g/0.85 = 16.7 g = 17 g (to 2 significant figures)