Why is it important that the primary standard chemical be non-hygroscopic and pure? Why is it important to dry the primary standard to a constant weight?

In the reading, yao chen-yuan describes traveling to deliver a message. why was he willing to risk danger to travelto tientsin? he wanted to the boxers with their cause

If 1.63 times 10 negative 4 of helium dissolves in 100.0g of water, what is the concentration in parts per million

Consider a culture medium on which only gram-positive organisms such as staphylococcus aureus colonies can grow due to an elevated nacl level. a yellow halo surrounds the growth, indicating the bacterium fermented a sugar in the medium, decreasing the ph as a result and changing the color of a ph indicator chemical. this type of medium would be referred to as a differential and enrichment culture.