The alkenes are compounds of carbon and hydrogen with the general formula cnh2n. if 0.561 gram of any alkene is burned in excess oxygen, what number of moles of h2o is formed?

To solve this problem, we must first examine the chemical formula of an alkene (CnH2n) and notice that there are 2 H atoms for every 1 C atom. 

Therefore this means that we can calculate the weight percent of Hydrogen (H) using the molecular formula: 

wt% H2 = 2n * (1.0g/mol H)/[2n*(1.0g/mol H) + n*(12.0g/mol C)) x 100 = 14.3 wt% H

So for any alkene with mass of 0.561g, this contains (0.143)x(0.561g) = 0.080 g H 

Now, we calculate the number of moles of H this is by dividing with the molar mass: 

moles H = (0.080g H) / (1.00 g/mol H) = 0.08 moles H. 

We know that there are 0.5 moles of water per mole of hydrogen, therefore the moles of H2O formed is: 

moles H2O = 0.5 * moles H = 0.5 * (0.08) = 0.04 moles H2O

0.04 moles water