Answer
$2.450\times10^{23}\,atoms$
Work Step by Step
Number of moles of Cu ($n$)=$\frac{\text{mass in grams}}{\text{molar mass}}$
$=\frac{25.85\,g}{63.546\,g/mol}=0.4068\,mol$
Number of atoms=$n\times N_{A}$
$=0.4068\times6.022\times10^{23}=2.450\times10^{23}\,atoms$
