An original sample of k-40 has a mass of 25.00 grams. after 3.9 × 109 years, 3.125 grams of the original sample remains unchanged. what
is the half-life of k-40?
(1) 1.3 × 109 y (3) 3.9 × 109 y
(2) 2.6 × 109 y (4) 1.2 × 1010 y

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.

Phosphoric acid is a triprotic acid ( =6.9×10−3 , =6.2×10−8 , and =4.8×10−13 ). to find the ph of a buffer composed of h2po−4(aq) and hpo2−4(aq) , which p value should be used in the henderson–hasselbalch equation? p k a1 = 2.16 p k a2 = 7.21 p k a3 = 12.32 calculate the ph of a buffer solution obtained by dissolving 18.0 18.0 g of kh2po4(s) kh 2 po 4 ( s ) and 33.0 33.0 g of na2hpo4(s) na 2 hpo 4 ( s ) in water and then diluting to 1.00 l.

What sources of error may have contributed to the percent yield not being 100 percent? think about things that may have led to inaccurate measurements or where mass of the product could have been lost if this experiment was conducted in a physical laboratory.