Enthalpy help

The standard enthalpy change of combustion of ethanol is -1371 kjmol-1. Calculate the mass of ethanol needed to heat 500cm3 of water by 25c

can someone help??

First I'd calculate the mass of water you have. Density = mass/volume, so mass = density * volume.

The density of water is 1000kgm^-3, so we convert the volume of water, 500cm³, into m³, which is 500 x 10^-6 or 5 x 10^-4 m³. Multiplied together, this gives a mass of about 0.5kg.

From here, you can calculate the energy needed to heat the water by 25 degrees using $Q=mc\Delta T$ where Q is the energy, m is the mass of water, c is the specific heat capacity of water, and $\Delta T$ is the change in temperature.

$Q = 0.5 * 4180 * 25 = 52250 J = 52.25 kJ$

Now, you know that the energy provided when combusting 1 mole of ethanol is 1371 kJ. We only need 52.25 kJ, which by dividing means we need about 0.038 moles of ethanol.

Finally, you can use $n = \frac {m}{m_R}$ rearranged to $m = nM_r$ where $M_r$ is the relative molecular mass of ethanol, about 46.

$m = 0.038 * 46 = 1.75g$ of ethanol

I think this is right anyway. Hope it helps!