A 5.0 g sample of spinal fluid contains 3.75 mg of glucose. What is the percent by mass of glucose?

1. Both masses must carry the same unit before dividing, so convert 3.75 mg to 0.00375 g. 2. Form the ratio: 0.00375 g / 5.0 g. 3. That equals 0.00075, and multiplying by 100 percent gives 0.075 percent. 4. As a rough check, 4 mg in 5000 mg is a little under one part in a thousand, so a bit under 0.1 percent. 5. Skipping the milligram to gram conversion gives 75 percent, which is wildly off and easy to spot. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.4 Other Units for Solution Concentrations_