1. hydrogen gas burns in air according to the equation below.
2h2(g) + o2(g) → 2h2o)
(a) calculate the standard enthalpy change. ah°298 for the reaction represented by the equation above
(the molar enthalpy of formation, ah for hom) is -285.8 kj molat 298 k.)

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.

Asample of aluminum foil contains 8.60 × 1023 atoms. what is the mass of the foil?

Nebulae are enormous clouds in outer space. they are made mostly of hydrogen gas, helium gas, and dust. some nebulae glow brightly, while others do not. the stars that people see are huge, bright balls of glowing gas. they are made mostly of hydrogen and helium. which statement correctly describes other ways in which nebulae and stars are different? a. stars can form inside a nebula but a nebula can never be produced by any star. b. a star always has a higher density than a nebula. c. stars can never form inside a nebula but a nebula can be produced by any star. d. a nebula always has a higher density than a star.