An unknown compound containing only C and H was burnt, yielding 10.2 g of CO2 and 6.3 g of H2O. With a molecular weight of about 30, what is the molecular formula?

Explanation:

Hello.

In this case, we can see that the mass of carbon of the unknown compound comes from the yielded mass of carbon dioxide, thus, we compute the moles of carbon as follows:

Moreover, the mass of hydrogen comes from the yielded water, therefore we can also compute the moles of water:

Then, to find the subscripts in the empirical formula, we divide by the moles of carbon as the smallest:

Whose molar mass is:

Thus, the ratio of the molecular formula to the empirical formula is:

Therefore, the molecular formula is twice the empirical formula:

Which is actually ethane.

Regards.

if you mean hydrogen and oxygen then water is the product

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.