If compound A reacts with B following first order kinetics with...

Given,
$\begin{cases} & & A + B \to P \\ t = 0 & & 7g \\ t = t & & 2g\end{cases}$

at constant volume

From formula of first order kinetics:
$⇒ t = \dfrac{2 . 303}{K}log\dfrac{{\left[A\right]}_{0}}{{\left[A\right]}_{t}}$

$= \dfrac{2 \cdot 303}{2 \cdot 011 \times {10}^{- 3}}log\dfrac{7}{2}$

$= \dfrac{2 \cdot 303 \times 0 . 544}{2 \cdot 011 \times {10}^{- 3}}$

$= 622 . 989$

$\approx 623$