<p>I would like to clear something up before I walk in to take the Physics B exam on Monday.</p>
<p>Say I'm given 200 mL of water, and told to find its mass. The unit conversion is simple: 200mL times 1g/mL equals 200 grams. The water weights 200 grams. Problem solved.</p>
<p>Now, lets change the method a little. 1g/mL equals 1000kg/L, right? Both are equivalent measures of water density. And 200mL equals 0.2L, right? So when we multiply the two we should get the same thing. Instead, the water now weights 200 kg.</p>
<p>I'm willing to bet there is a very good explanation to this, but I'm yet to find it.</p>
<p>In the metric system, you have your base units (Liters, grams, meters, ect) and you have your prefixes. The prefixes represent powers of 10 which are multiplied with the base unit. The prefix “milli” means one thousandth, which means that a milliliter is one thousandth of a Liter. Since a mL is one thousandth of a L, the power of 10 here is 10^-3.</p>
<p>1 mL = 1 L x 10^-3
1mL = (1/1000)L ----> 1000mL = 1 L</p>
<p>That doesn’t really answer my question, but thanks :)</p>
<p>Your problem is that 1g/mL doesn’t equal 1000kg/L. 1g = .001kg, and 1mL = .001L, so it’s .001kg/.001mL = 1kg/L</p>
<p>Looks like I got the wrong definition of a liter.</p>