Iron (II) has a positive two charge: .

Phosphate has a negative three charge: .

The initial compound must be constructed to cancel these charges. The dissociation is: .

Calcium is in the second group of the periodic table, and is therefore going to have a  oxidation number. Hydroxide ions have a charge. Calcium hydroxide will have the formula .

Chloride ions have a charge and hydrogen ions have a charge. The formula for hydrochloric acid is .

On the products side, water has the formula and calcium chloride has the formula .

Now that we know all of the formulas, we can write our reaction:

In order to balance the chloride atoms, we need to add coefficients.

The balanced reaction for the combustion of pentane is:

When balanced, oxygen gas has a coefficient of eight.

To balance the equation, it is easiest to leave oxygen and hydrogen for last. This means we should start with carbon.

Now that carbon is balanced, we can look at hydrogen.

Finally, we can balance the oxygen.

The final reaction uses five carbon atoms, twelve hydrogen atoms, and sixteen oxygen atoms per side.

The balanced chemical equation is:

The mole ratio of iron oxide to solid iron is 1:2. You can set up the following proportion to solve:

Potassium is a Group I element, so to get to a filled valence shell, it will lost one electron, yielding .

Arsenic is a Group 5 element, so it needs to gain three electrons to obtain a filled valence shell, yielding .

In order to balance out the charges, the resultant salt will be .

First, we must know what ferrous sulfate is. Ferrous refers to , and sulfate has the formula . When we combine the two together we get .

Calcium is a divatent cation and iodide is a monovalent anion, so their salt is . The ion exchange reaction is then:

The net ionic equation is derived by removing all spectator ions from the total ionic equation (in which all ions are listed). To put it another way, the net ionic equation involves only the ions that participate in a reaction which, in this case, is the precipitation of barium sulfate.

Begin by writing all aqueous compounds in their dissociated (ionic) forms.

Cancel out any ions that appear in equal quantities on both sides of the equation. In this case, we can cancel the nitrate and potassium ions.

This is our net ionic equation.

Combustion is the chemical reaction of a hydrocarbon with molecular oxygen, and it always produces carbon dioxide and water. Knowing the reactants and products, the unbalanced equation must be: 

We start by balancing the hydrogens. Since there are 10 on the left and only 2 on the right, we put a coefficient of 5 on water.

Similarly, we balance carbons by putting a 4 on the carbon dioxide.

To find the number of oxygens on the right, we multiply the 4 coefficient by the 2 subscript on O (which gets us 8 oxygens) and then add the 5 oxygens from the 5 water molecules to get a total of 13. The needed coefficient for  on the left would then have to be 13/2.

Because fractional coefficients are not allowed, we mutiply every coefficient by 2 to find our final reaction:

When we check the activity series, it is fairly easy to see that aluminum metal is more reactive than zinc metal. So, in this case, the two metals undergo a redox reaction, where the aqueous  is reduced to solid , and the solid  is oxidized to aqueous . These charges are the common oxidation states for zinc and aluminum and should be memorized.

Because  is the new species, it bonds with 3  ions. The unbalanced equation is:

We note that there are 2 chlorine atoms on the left and 3 chlorine atoms on the right. To balance, we use a 3 coefficient on the left and a 2 coefficient on the right. This gives a total of 6 chlorine atoms on eahc side.

However, now we have also increased the amounts of zinc and aluminum. We copy the necessary coefficients to balance those—2 for aluminum on the left, 3 for zinc on the right—and we are done:

Balancing reactions is best acheived by using a stepwise approach.  It is useful to work through each atom making sure it is present in a balanced fashion on both sides of the equation.  Starting with Fe, Fe₂O₃ is the only molecule with Fe present on the left side of the equation and FeCl₃ is the only molecule with Fe present on the right side of the equation.  Thus the molar ratio of Fe₂O₃ to FeCl₃ must be 1:2.  Moving on to O, the O on the left side of the equation exists as Fe₂O₃ and H₂O on the right.  The molar ratio of Fe₂O₃ to H₂O is therefore 1:3.  Cl exists in HCl on the left and FeCl₃ on the right. Thus the molar ratio of HCl to FeCl₃ must be 3:1. To ensure that these molar ratios are maintaned the balanced formula is then determined to be Fe₂O₃ + 6HCl -> 2FeCl₃ + 3H₂O