Final answer:
The density of the substance is 750 g/cm³, but without the volume of the oxygen tank, we cannot conclusively say if it will explode at 398°C. Typically, raising the temperature while keeping volume constant increases pressure, and if it exceeds 25 atm, the tank would be at risk of exploding.
Step-by-step explanation:
The density of a substance is calculated by dividing its mass by its volume. For a substance with a mass of 0.90 kg and a volume of 1.2 mL, you can use the formula density = mass / volume. Given that 1 kg is equal to 1,000,000 mg, first, we convert 0.90 kg to mg by multiplying by 1,000,000, which gives us 900,000 mg. Then, we divide this by 1.2 mL to find the density, resulting in a density of 750,000 mg/mL or 750 g/cm³ since 1 mL is equivalent to 1 cm³.
In regards to the second question involving the oxygen tank, we can apply the ideal gas law to predict whether the tank will explode at a raised temperature. However, without specific volume data, an exact answer cannot be given. We know that if the temperature increases while the volume doesn't change, then the pressure will also increase. If this increase causes the pressure to exceed 25 atm, the tank will explode. The student would need to compute this using the ideal gas law formula P1/T1 = P2/T2, where P is pressure, T is temperature (in Kelvin), and the subscripts 1 and 2 refer to initial and final conditions. A full calculation would require the volume of the tank, which is not provided.