# The Half-life element X is 15 min. What weight of X was original present in a sample of 40 grams is left after 1 hour? ?

AP Chem Question:

Choice answers are:

A) 160 grams

B) 320 grams

C) 640 grams

D) 1280 grams

### 6 Answers

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- 冷眼旁觀Lv 72 months agoFavorite Answer
1 hour = 4 half-lives

40 g = mₒ × (1/2)⁴

Original mass, mₒ = 40 × 2⁴ g = 640 g

The answer: C) 640 g

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- Anonymous2 months ago
4 half lives, 1/16 of the original amount.

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- Jim MoorLv 72 months ago
A = A₀ (1/2)^(t/h)

40 = A₀ (1/2)^(60m/15m)

40 = A₀ (1/2)^(4) = A₀ (1/16

A₀ = 40 *16/1 = 640g ANswer C.

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- Old Science GuyLv 72 months ago
N = No e^(-0.693*T/Tk)

40 = No * e^(-0.693 * 60/15) = No * 0.0625

No = 640 g

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- MorningfoxLv 72 months ago
15 minutes ... half is left.

30 minutes ... 1/4 is left

45 minutes ... 1/8 is left

60 minutes ... (can you figure this part ?)

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