Answer: We can use the combined gas law to solve this problem:
(P₁V₁/T₁) = (P₂V₂/T₂)
where P is pressure, V is volume, and T is temperature in Kelvin.
We know that P₁ = P₂ (the pressure is assumed to be constant), and we are given V₁, T₁, and V₂. We can solve for T₂:
(P₁V₁/T₁) = (P₂V₂/T₂)
T₂ = (P₂V₂/T₁) * (T₁/P₁V₁)
We need to convert the initial temperature from Celsius to Kelvin:
T₁ = 543 + 273 = 816 K
Substituting the values:
T₂ = (1 atm * 86 mL / 816 K) * (51.1 mL / 1 atm * 86 mL)
T₂ = 0.0629 * 51.1 * 1000 = 3217 K
Therefore, the marshmallow would need to be heated to a temperature of 3217 K for its volume to change from 86 mL to 51.1 mL.
Enjoy (: