$$\begin{aligned}&\text { Ratios of weight of zinc and copper in the alloy }=7: 9 \\&\text { Let the weight of zinc in the alloy }=7 x \\&\text { Weight of copper alloy in the alloy }=9 x \\&\text { Total weight of the alloy }=8 \mathrm{~kg} \\&\Rightarrow 7 x+9 x=8 \mathrm{~kg} \\&\Rightarrow 16 x=8 \mathrm{~kg} \\&\Rightarrow x=\frac{8}{16} \mathrm{~kg}=\frac{1}{2} \mathrm{~kg}\end{aligned}$$
Weight of copper in the alloy $$=9 x$$
$$\begin{aligned}&=9 \times \frac{1}{2} \\&=\frac{9}{2} \mathrm{~kg} \\&=4 \frac{1}{2} \mathrm{~kg}\end{aligned}$$
$$\therefore \textbf{Weight of copper in the alloy is } 4 \frac{1}{2} \mathrm{~kg}$$