Active mass of solids shouldn't necessarily be unity I've read that active mass of solids and pure liquids is considered unity. I'm absolutely sure there is some misunderstanding. I know that concentration of solids doesn't change during the course of reaction, but it shouldn't necessarily be unity either. Consider graphite for example. It's density is $2260\pu{g/L}$
$$\text{[C]}=\frac{n_C}{V_C}$$ where $n_C$ is the number of moles of the carbon sample and $V_C$ is the volume of that sample. $$\text{[C]}=\frac{w}{M\times V_C} $$ $$\text{[C]}=\frac{\rho_C}{M}=\frac{2260\pu{g/L}}{12\pu{g/mol}}\\=188.3\pu{mol/L}$$This concentration will be constant throughout a reaction if the phase doesn't change and temperature change is quite small. But this clearly is not unity. Then why is the concerntration of solids considered to be unity?
Rather, active mass of solids need not necessarily be unity, but arbitrarily is unity with advantage. You do not have to know the molar mass nor density.
The common thermodynamic expressions like $K = \frac{[C][D]}{[A][B]}$
or kinetic ones like $\frac{\mathrm{d}[C]}{\mathrm{d}t} = k \cdot [A] \cdot [B]$
should be more exactly written as $K = \frac{a_\mathrm{C}] \cdot a_\mathrm{D}}{a_\mathrm{A} \cdot a_\mathrm{B}}$ or $\frac{\mathrm{d}[C]}{\mathrm{d}t} = k \cdot a_\mathrm{A} \cdot \cdot a_\mathrm{B}$
where $a_\mathrm{X}$ is unitless quantity called activity, defined by the chemical potential of the substance $ \mu_\mathrm{A} = \left( \frac {\partial G}{\partial n_\mathrm{A}} \right)_{T,p}$ by formula:
$$\mu_\mathrm{A} = RT \ln {a_\mathrm{A, unitless}}$$
If we implement the standard chemical potential, we can define unitless activity as ratio of the activity with unit and the standard activity with the unit value.
$$\mu_\mathrm{A} = \mu_\mathrm{A}^{\circ} + RT \ln \left( \frac {a_\mathrm{A}}{a_\mathrm{A}^{\circ}} \right)$$
where $a_\mathrm{A}^{\circ}$ is the activity at the standard chemical potential $\mu_\mathrm{A}^{\circ}$. We can consider the activity $a_\mathrm{A} = x\ \text{"whatever unit"}$ and the standard activity $a_\mathrm{A}^{\circ} = 1 \ \text{"whatever unit"}$.
If we arbitrarily choose $a^{\circ} = \pu{1 mol/L}$, then $a$ then more or less closely follow the respective concentration [A].
Chemical potential of the pure solid substance $\mu_\mathrm{A}^{\circ}$ does not depend on the amount of substance, so we arbitrarily choose $a^{\circ} = x_\mathrm{A}^{\circ}=1.0$ and $a$ then more or less follows the  molar fraction of substance $x_\mathrm{A}$.