we now understand that when we are looking at solutions the easiest way to discuss the concentration of the solution or how much solute is in a solution is to use molarity as it can apply to solutions of any volume molarity tells us the moles of solute per liter of solution and since molarity equals moles per liter that means that if we have any two of these quantities concentration moles or liters we can always calculate the third all we need to do is rearrange the equation for example we can take this equation molarity equals moles over liters and simply multiply both sides by liters to get concentration times liters equals moles so if we need to solve for moles we would use this equation if we then take that equation and divide both sides by concentration we get liters equals moles over concentration so if we need to solve for volume we would use this equation now we can see that we have three different equations that we can use depending on which component we wish to solve for let's say we had a solution of salt water and we knew that it was 2.5 molar and had a volume of 500 milliliters how many moles of salt are in the solution well since we are solving for moles let's take this equation here 2.5 moles per liter times 0.5 liters gives us 1.25 moles we just plug in the values we know and do the arithmetic conversely let's say we make another solution using 0.65 moles and we want the solution to be 1.8 molar what must the volume be in this case we could use this equation 0.65 moles over 1.8 moles per liter gives us 0.361 liters or 361 milliliters so now we understand how molarity moles and volume relate to one another and that given any two of the three we can easily calculate the third with just a little bit of algebra and arithmetic