Question

An unknown compound contains only carbon, hydrogen, and oxygen (cxhyoz). combustion of 4.50 g of this compound produced 6.60 g of carbon dioxide and 2.70 g of water. how many moles of hydrogen, h, were in the original sample

Answer 1

The number of moles of Hydrogen H  that were in the original sample is

0.30 moles

calculation

 find the formula of the  unknown compound

•       write the equation for reaction

               CxHyOz  + O2 →  CO2 + H2O

• find the moles of CO2 produced

   moles= mass/molar mass

        =  6.60g/44 g/mol=0.15 moles

• find the moles of CxHyOz

• moles of CO2= moles of CxHyOz  therefore the moles of CxHyOz =0.15 moles

• find the molar mass of CxHyOz

  molar mass= mass/  moles

 =  4.50/0.15=  30 g/mol

• determine value  the for X,Y,Z

 C =x(12 g/mol)  if  x=1  then c= 12  g/mol

  H= y (1 g/mol) if Y=1 then H=1 g/mol

   O=Z(16g/mol) if O=16 then O= 16g/mol

 the formula of the compound from the value of X,y and  z above = CHO

• the MW of CHO= (12 g/mol + 1 g/mol +16g/mol)= 29 g/mol

• since the molar mass of the compound was 30  g/mol 1 more atom of oxygen is required, therefore the formula of unknown compound= CH2O

• from the moles of CH2O= 0.15 moles  and there are two atoms of H in CH2O therefore the moles of H atom = 0.15 x2=0.30 moles