[Music] in this video we're going to be looking at this equation here which links the number of moles of an aqueous substance to its concentration and its volume and you need to remember that we measure volume in decimeters cubed and concentration in moles per decimeter cubed which we normally write as moles so mol and then a slash decimeters cubed because this forward slash means per you might also sometimes see the formula written like this with n for moles c for concentration and v for volume so if we had 800 centimeters cubed of sodium hydroxide at a concentration of 0.2 moles per decimeter cubed then to find out how many moles we had in total we would just divide our volume by 1000 to make it 0.8 decimeters cubed and then do volume times concentration so 0.8 times 0.2 which gives us 0.16 moles or for another one let's imagine that we have 1.5 decimeters cubed of hydrochloric acid and we're told that it contains 0.6 moles of hydrochloric acid what must the concentration be well if we look at our equation concentration is moles divided by volume and as they're both already in the right units we just do 0.6 divided by 1.5 which gives us a concentration of 0.4 moles per decimeter cubed now a more complicated scenario is when we're given an equation like this one here and we're asked to find an unknown concentration or volume for example if we knew that 30 centimeters cubed of 0.5 mole per decameter cubed potassium hydroxide reacted completely with 25 centimeters cubed of sulfuric acid we should be able to work out the concentration of the sulfuric acid before we do anything else though it's worth writing out clearly what we already know from the question for the potassium hydroxide its volume is 30 centimeters cubed and its concentration is 0.5 moles per decimeter cubed and for the sulfuric acid we know its volume is 25 centimeters cubed then because this is a chemistry calculation we know that we're going to need all of our volumes in decimeters cubed and not centimeters cubed so we divide 30 by 1000 to get 0.03 decimeters cubed and 25 by 1000 to get 0.025 cubed now the basic idea with all of these types of questions is first to work out how many moles of the known substance we have then second use the molar ratio to work out how many moles of the unknown substance we have and finally use that number of moles to work out the concentration of the unknown substance so our first step is to calculate how many moles of potassium hydroxide we have which we can do by multiplying this concentration by its volume so we do 0.5 times 0.03 meaning we have 0.015 moles of potassium hydroxide then for our second step we need to find the molar ratio between potassium hydroxide and sulfuric acid which we can do by looking at the numbers in front of each of them and remember that if there isn't a number before the substance like for the sulfuric acid then we just read it as a one so the molar ratio here is two to one which just means that for every two moles of potassium hydroxide we'll have one mole of sulfuric acid so because we just worked out that we have 0.015 moles of potassium hydroxide we're gonna have to divide that by two to find out how many moles of sulfuric acid it reacted with giving us 0.0075 moles of sulfuric acid and finally we can move on to our third step which is where we actually calculate the concentration using the number of moles that we just found and the volume that we were given in the question which remember we converted here to 0.025 decimeters cubed so we take our 0.0075 moles of sulfuric acid and divide it by the volume of 0.025 which tells us that the concentration of sulfuric acid is 0.3 moles per decimeter cubed and that is our final answer to the question anyways that's everything for this video so hope that all made sense and we'll see you again soon