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The enthalpy of combustion of methanol (GFM = 32.0 g) is −726 kJ mol−1 - Scottish Highers Chemistry - Question 7 - 2023

Question 7

The enthalpy of combustion of methanol (GFM = 32.0 g) is −726 kJ mol−1. What mass of methanol has to be burned to produce 145.2 kJ?

Worked Solution & Example Answer:The enthalpy of combustion of methanol (GFM = 32.0 g) is −726 kJ mol−1 - Scottish Highers Chemistry - Question 7 - 2023

Step 1

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Answer

To find out how many moles of methanol are needed to produce 145.2 kJ, we can use the enthalpy of combustion:

The enthalpy of combustion of methanol is −726 kJ/mol. From this, we can calculate the moles needed:

$n = \frac{\text{Energy required}}{\text{Enthalpy change}} = \frac{145.2 \text{ kJ}}{726 \text{ kJ/mol}} = 0.199 \text{ mol}$

Step 2

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Answer

Now, we can convert the moles of methanol into grams using its molar mass:

The molar mass of methanol is 32.0 g/mol. Thus, the mass can be calculated as:

$\text{Mass} = n \times \text{Molar mass} = 0.199 \text{ mol} \times 32.0 \text{ g/mol} = 6.368 \text{ g}$

Rounding to three significant figures, the mass of methanol needed is approximately 6.4 g.

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