Question

the (balanced) reaction for the combustion of ethanol (c₂h₅oh) is shown below:
c₂h₅oh(l) + 3 o₂(g) → 2 co₂(g) + 3 h₂o(g) δh = -1240 kj
if 575 kj of heat are released, what mass of ethanol mw = 46.062 g mol⁻¹ is burned?
○ 21.4 g
○ 46.1 g
○ 0.464 g
○ 10.7 g

Explanation:

Step1: Find moles of ethanol

The reaction releases 1240 kJ per mole of ethanol. So moles of ethanol ($n$) = heat released / heat per mole = $575\space kJ / 1240\space kJ/mol$.
$n = \frac{575}{1240} \approx 0.4637\space mol$

Step2: Calculate mass of ethanol

Mass ($m$) = moles × molar mass. Molar mass of ethanol is 46.062 g/mol.
$m = 0.4637\space mol × 46.062\space g/mol \approx 21.4\space g$

Answer:

21.4 g (corresponding to the option "21.4 g")