Question

the percent by mass of copper(ii) in cu(no2)2 is %.

Explanation:

Step1: Calculate molar masses

The molar mass of Cu is $M_{Cu}=63.55\ g/mol$. The molar mass of $N$ is $M_N = 14.01\ g/mol$, the molar mass of $O$ is $M_O=16.00\ g/mol$. For $Cu(NO_2)_2$, the molar - mass $M$ is calculated as follows:
\[

$$\begin{align*} M&=M_{Cu}+2\times(M_N + 2\times M_O)\\ &=63.55+2\times(14.01 + 2\times16.00)\\ &=63.55+2\times(14.01 + 32.00)\\ &=63.55+2\times46.01\\ &=63.55 + 92.02\\ &=155.57\ g/mol \end{align*}$$

\]

Step2: Calculate mass - percent of Cu

The mass - percent of Cu in $Cu(NO_2)_2$ is given by the formula $\text{Mass}\%=\frac{\text{Mass of Cu in compound}}{\text{Molar mass of compound}}\times100\%$.
\[

$$\begin{align*} \text{Mass}\%&=\frac{63.55}{155.57}\times 100\%\\ &\approx 40.85\% \end{align*}$$

\]

Answer:

$40.85$