You must have taken the same chemistry class that I took. [smile icon]
Unfortunately, we are not in general chemistry here, but we are in Jim Humbles Miracle-Mineral-Supplement chemistry...
Normally, you would make sure your citric acid is anhydrous and make the solution based upon weight. However, Jim Humble has instructed people to mix the citric acid by volume.
In this case, the difference is not that great. If a person doesn't have a gram scale, they can get reasonably close mixing the citric acid by volume.