There is an unstable ratio of protons and neutrons. Because protons are positively charged but neutrons have no charge, an increase in the number of protons means there needs to be an increase in the number of neutrons to "bind" the nucleus together. This is because like charges repel, so the protons will repel each other, and if there aren't enough neutrons to act as "glue" to hold the nucleus together, the nucleus will break apart.
Answer:

Explanation:
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In this case, since the average rate of reaction is computed as a change given by:
![r=\frac{\Delta [NH_4NO_2 ]}{\Delta t}](https://tex.z-dn.net/?f=r%3D%5Cfrac%7B%5CDelta%20%5BNH_4NO_2%20%5D%7D%7B%5CDelta%20t%7D)
In such a way, given the concentrations at the specified times, we plug them in to obtain:

Whose negative sign means the concentration decreased due to the decomposition.
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STP condition mean we have P=1 atm. T=273K. R=ideal gas constant, but make sure you use the one that has the same units of pressure, temperature that you are using. In this case R=0.0821 L*atm K^-1mol^1. You are provided with n=2.1 moles.
V=nRTP
Input your values and solve.
The brick is solid because it's not hollow or containing spaces or gaps