Video Transcript
A 5.0-gram sample of spinal fluid contains 3.75 milligrams, 0.00375 grams, of glucose. What is the percent by mass of glucose in spinal fluid?
To calculate a percent, we divide the part in question by the whole, then multiply by 100 percent. To calculate a percent by mass, we simply need to use the mass of the part and the mass of the whole in the equation. Looking at the question, we need to determine the percent by mass of glucose in spinal fluid. Glucose is the part we want to know about, and spinal fluid is the whole. We are told that a 5.0-gram sample of spinal fluid contains just 3.75 milligrams of glucose.
Now that we know the mass of the part and the whole, we can calculate the percent by mass of glucose in spinal fluid. But we need to be careful here. A percentage is a unitless value. This means that the mass units must be the same in order for the units to cancel. So, instead of using the mass of glucose in milligrams, we should use the mass provided in grams. Now the mass units will cancel, and we’ll be left with a unitless percent. Performing the calculation gives us a percentage of 0.075 percent. So the percent by mass of glucose in the sample of spinal fluid is 0.075 percent.
