Final answer:
The quantity of heat evolved when 15g of CH4 is burned completely in air is 832.624 kJ, calculated by converting the mass of CH4 to moles and using the provided enthalpy change of combustion.
Step-by-step explanation:
The quantity of heat evolved when 15g of CH4 is completely burned in air can be calculated using the enthalpy change provided (ΔH=-890 kJ/mol). First, we need to convert the mass of methane to moles using its molar mass (16.04 g/mol), then apply the stoichiometric relationship between the moles of methane burned and the heat released.
To find the moles of CH4, we use the formula:
moles = mass (g) / molar mass (g/mol).
For 15g of CH4, the calculation is 15g / 16.04 g/mol = 0.935 mol.
Given that the combustion of 1 mol of methane releases 890.4 kJ of heat, the total heat evolved for 0.935 mol can be calculated by multiplying the moles by the enthalpy change per mole:
Heat evolved = moles of CH4 x ΔH = 0.935 mol x 890.4 kJ/mol = 832.624 kJ.