How many photons of light are emitted when the electrons in 10 hydrogen atoms drop from energy level 5 to 3?

Someone help explain how to do this!!

What is the ph of a solution that has a hydrogen ion concentration of 1.0 * 10 -9 m?

An engineer decides to use a slightly weaker material rather than a stronger material, since she knows that the stronger material can break suddenly. this is an example of what? a choosing a material that will show warning before it fails b using composite materials that combine strength c using a material for multiple applications d using design techniques that increase efficiency and reduce cost

Acompound has the molecular formula c3h8. which class of organic compounds does it belong to?