What does the letter r symbolize in a structural formula, such as rcooh?

R means there's a number of carbon in the chain.

Explanation:

For example RCOOH means that after x number of carbon, the chain ends with COOH.

a  solution  is always transparent, light passes through with no scattering from solute particles which are molecule in size. the  solution  is homogeneous and does not settle out. a colloid is intermediate between asolution  and a  suspension. while asuspension  will separate out a colloid will not.

frequency, z, and the collision density, z, in carbon dioxide (co2), r 180 pm at 30 °c and 120 kpa