when dissolved in water a strong base like potassium hydroxide will dissociate completely in solution to form hydroxide ions potassium hydroxide is an example of a group 1a metal hydroxide other examples include lithium hydroxide and sodium hydroxide group 2a metal hydroxides are also considered to be strong bases for example calcium hydroxide is a group 2a metal hydroxide and so is strontium hydroxide let's do a problem with a group 1a metal hydroxide sodium hydroxide let's say the ph of the solution is 13.00 and our goal is to calculate the initial concentration of sodium hydroxide first we write out the dissolution equation solid sodium hydroxide dissociates completely in water to form sodium cations and hydroxide anions in solution looking at the balanced equation there's a one in front of sodium hydroxide and a one in front of hydroxide ions therefore the concentration of hydroxide ions is equal to the initial concentration of sodium hydroxide and we can find the concentration of hydroxide ions in solution from the ph at 25 degrees celsius the ph plus the poh is equal to 14.00 so we can plug the ph into our equation which gives us 13.00 plus the poh is equal to 14.00 so the poh of the solution is equal to 1.00 and the poh is equal to the negative log of the concentration of hydroxide ions so we can plug the poh into this equation which gives us 1.00 is equal to the negative log of the concentration of hydroxide ions to solve for the concentration of hydroxide ions in solution first we move the negative sign to the left side which gives us negative 1.00 is equal to the log of the concentration of hydroxide ions to get rid of the log we take 10 to both sides so the concentration of hydroxide ions in this solution is equal to 10 to the negative 1.00 which is equal to 0.10 molar because sodium hydroxide is a strong base that dissociates completely in solution to form hydroxide ions if the concentration of hydroxide ions in solution is 0.10 molar so is the initial concentration of sodium hydroxide now let's do a problem with a group 2a metal hydroxide let's say the initial concentration of a solution of calcium hydroxide is .0010 molar and our goal is to find the ph of the solution at 25 degrees celsius calcium hydroxide is a strong base that dissociates completely in solution to form calcium 2 plus ions and hydroxide anions and looking at the mole ratios in this dissolution equation there's a one in front of calcium hydroxide a one in front of calcium two plus and a two in front of hydroxide ions since the mole ratio of calcium hydroxide to calcium is one to one if the initial concentration of calcium hydroxide is .0010 molar that's also the concentration of calcium ions so it's 0.0010 molar in solution the mole ratio of calcium hydroxide to hydroxide ions is one to two so if the initial concentration of calcium hydroxide is point zero zero one zero molar the concentration of hydroxide ions in solution is twice that concentration so two times point zero zero one zero molar is equal to point zero zero two zero molar now that we know the concentration of hydroxide ions we can calculate the ph of the solution one way to calculate the ph is to first find the poh of the solution and poh is equal to the negative log of the concentration of hydroxide ions so we can plug in the concentration of hydroxide ions into our equation which gives us the poh of the solution is equal to the negative log of point zero zero two zero and when you do the calculation the poh is equal to 2.70 notice since we have two significant figures for the concentration we need two decimal places for rpoh so to find the ph we know that ph plus poh is equal to 14.00 at 25 degrees celsius so we can plug in the poh of 2.70 and that gives us ph plus 2.70 is equal to 14.00 so the ph of the solution is equal to 11.30