I think the major difference is that 0.5 is exactly equal to a half. The other 0.555555 is equal to a half plus five hundredths plus thousandths plus five ten thousandths plus five hundred thousandths, plus five millionths and so forth; and therefore it is greater than 0.5. In other words 0.555555 is called a recurring decimal.
H2O and H3O+ are a pair. The conjugate base of an acid is the base that is formed when the acid has donated a hydrogen ion. The conjugate acid of a base is the acid that forms when base accepts a hydrogen ion.
The number of years required for 1/4 cobalt-60 to remain after decay is calculated as follows
after one half life 1/2 of the original mass isotope remains
after another half life 1/4 mass of original mass remains
therefore if one half life is 5.3 years then the years required
= 2 x 5.3years = 10.6 years
Oxygen had been produced by several chemists prior to its discovery in 1774, but they failed to recognize it as a distinct element. Joseph Priestley and Carl Wilhelm Scheele both independently discovered oxygen, but Priestly is usually given credit for the discovery. They were both able to produce oxygen by heating mercuric oxide (HgO). Priestley called the gas produced in his experiments 'dephlogisticated air' and Scheele called his 'fire air'. The name oxygen was created by Antoine Lavoisier who incorrectly believed that oxygen was necessary to form all acids.