Question #fee47

2 Answers

You balance an equation with parentheses the same way as with other equations.

You must remember to multiply the subscripts of atoms inside the parentheses by the subscripts outside the parentheses.

Explanation:

For example, balance the equation:

"Fe(NO"_3)_2 + "Na"_3"PO"_4 → "Fe"_3("PO"_4)_2 + "NaNO"_3Fe(NO3)2+Na3PO4Fe3(PO4)2+NaNO3

The standard method is to balance by inspection:

  • First balance all atoms other than "O"O and "H"H.
  • Then balance "O"O.
  • Then balance "H"H.

Start with the most complicated formula. Here, it is "Fe"_3("PO"_4)_2Fe3(PO4)2:

We have "three Fe"three Fe atoms on the right, so we need to have "three Fe"three Fe atoms on the left hand side of the equation.

color(red)(3)"Fe(NO"_3)_2 + "Na"_3"PO"_4 → color(red)(1)"Fe"_3("PO"_4)_2 + "NaNO"_33Fe(NO3)2+Na3PO41Fe3(PO4)2+NaNO3

We have "two P"two P atoms on the right side (one "P"P atom, which is inside the parentheses, is multiplied by the 2 outside). So we need "two P"two P atoms on the left hand side of the equation.

color(red)(3)"Fe(NO"_3)_2 + color(teal)(2)"Na"_3"PO"_4 → color(red)(1)"Fe"_3("PO"_4)_2 + "NaNO"_33Fe(NO3)2+2Na3PO41Fe3(PO4)2+NaNO3

Now we have "six N"six N atoms on the left hand side (1 "N"N atom inside the parentheses is multiplied by the 2 outside and again by the 3 in front of the whole formula). We need "six N"six N atoms on the right hand side of the equation.

color(red)(3)"Fe(NO"_3)_2 + color(teal)(2)"Na"_3"PO"_4 → color(red)(1)"Fe"_3("PO"_4)_2 + color(blue)(6)"NaNO"_33Fe(NO3)2+2Na3PO41Fe3(PO4)2+6NaNO3

We have "6 Na"6 Na atoms on the left and "6 Na"6 Na atoms on the right, so "Na"Na is balanced.

Now we want to balance "O"O atoms. The total number of "O"O atoms on the left hand side of the equation is 26. So we have 8 + 18 = 26 "O"O atoms on the right. "O"O atoms are balanced.

The balanced equation is:

3"Fe(NO"_3)_2 + 2"Na"_3"PO"_4 → "Fe"_3("PO"_4)_2 + 6"NaNO"_33Fe(NO3)2+2Na3PO4Fe3(PO4)2+6NaNO3

Jun 10, 2015

We balance by inspection, but we treat each group inside parentheses as if it were a single atom.

This is sometimes called the "EASY" way.

Explanation:

For example, balance the equation:

"Fe(NO"_3)_2 + "Na"_3"PO"_4 → "Fe"_3("PO"_4)_2 + "NaNO"_3Fe(NO3)2+Na3PO4Fe3(PO4)2+NaNO3

The groups inside parentheses behave as a unit, so we treat the "NO"_3NO3 as if it were an atom "X"X, and "PO"_4PO4 at if it were an atom "Y"Y.

Then the equation becomes

"FeX"_2 + "Na"_3"Y" → "Fe"_3"Y"_2 + "NaX"FeX2+Na3YFe3Y2+NaX

We begin with "Fe"_3"Y"_2Fe3Y2 and balance "Fe"Fe.

color(red)(3)"FeX"_2 + "Na"_3"Y" → color(red)(1)"Fe"_3"Y"_2 + "NaX"3FeX2+Na3Y1Fe3Y2+NaX

We have "2 Y"2 Y groups on the right hand side, so we need "2 Y"2 Y groups on the left hand side of the equation.

color(red)(3)"FeX"_2 + color(teal)(2)"Na"_3"Y" → color(red)(1)"Fe"_3"Y"_2 + "NaX"3FeX2+2Na3Y1Fe3Y2+NaX

We have "6 X"6 X groups on the left hand side. We need "6 X"6 X groups on the right hand side of the equation.

color(red)(3)"FeX"_2 + color(teal)(2)"Na"_3"Y" → color(red)(1)"Fe"_3"Y"_2 + color(blue)(6)"NaX"3FeX2+2Na3Y1Fe3Y2+6NaX

All atoms and groups are now balanced.

The balanced equation is:

color(red)(3"FeX"_2 + 2"Na"_3"Y" → "Fe"_3"Y"_2 + 6"NaX")3FeX2+2Na3YFe3Y2+6NaX

Now all we have to do is re-insert the original groups represented by "X"X and "Y"Y. We get

"3Fe(NO"_3)_2 + "2Na"_3"PO"_4 → "Fe"_3("PO"_4)_2 + "6NaNO"_33Fe(NO3)2+2Na3PO4Fe3(PO4)2+6NaNO3