I'm having a bit of trouble identifying which equation is not balanced.  can you help? 2C2H6 + 7O2 -----> 4CO2 + 6H2O      (b) C3H8 + 5O2 -----> 3CO2 + 4H2O      (c) C6H18 + 10O2 -> 5CO2 + 9H2O + 2CO + C      (d) C5H12 + 8O2 -----> 5CO2 + 6H2O

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Balanced is another word for equal.  Are the number of elements on each side of the equation (arrow) equal?

To determine the number of atoms you have to take into account coefficients (numbers in front) and subscripts (numbers below the element).  The numbers in front (coefficients) are multiplied throughout the substance.    Doing this you will find that equation c is unbalanced because the number of carbons are not equal.  On the left side of the arrow (reactants) there are 6 C; on the right side of the arrow (products) there are 8 C.  Not equal, therefore not balanced.

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Hello!

This question is more about math than chemistry: we have to carefully count atoms of each type on both sides of equations. Here we see three types of atoms: C (carbon), H (hydrogen) and O (oxygen). Take into account that index n after atom symbol means that the molecule has n such atoms, and no index means 1 atom. This number must be multiplied with the number of molecules.

a) 2C2H6 + 7O2 -----> 4CO2 + 6H2O
C: 2*2 + 0 = 4*1 + 0 , H: 2*6 + 0 = 0 + 6*2, O: 0 + 7*2 = 4*2 + 6*1.
All these equations are true, so equation is balanced.

b) C3H8 + 5O2 -----> 3CO2 + 4H2O
C: 1*3 + 0 = 3*1 + 0, H: 1*8 + 0 = 0 + 4*2, O: 0 + 5*2 = 3*2 + 4*1.
Again balanced.

c) C6H18 + 10O2 -----> 5CO2 + 9H2O + 2CO + C.
C: 1*6 + 0 = 5*1 + 0 + 2*1 + 1, or 6 = 8 -- false!
So this equation is unbalanced.
(it will be balanced if we omit 1CO and C, check)

d) C5H12 + 8O2 -----> 5CO2 + 6H2O
C: 1*5 + 0 = 5*1 + 0, H: 1*12 + 0 = 0 + 6*2, O: 0 + 8*2 = 5*2 + 6*1.
Balanced.

The answer: c) is unbalanced, a), b), d) are balanced.

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