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kkurt [141]
3 years ago
14

How many substances does a pure piece of matter contain? A. 0 B. 1 C. 2 D. 3

Chemistry
1 answer:
Rama09 [41]3 years ago
4 0
B. 1. Think of pure grade alcohol is 100% alcohol
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Promethium-147 is sometimes used in luminescent paint. It has a half-life of 956.3 days. If 250 grams (g) of promethium-147 is u
Fantom [35]

Answer:

% = 76.75%

Explanation:

To solve this problem, we just need to use the expressions of half life and it's relation with the concentration or mass of a compound. That expression is the following:

A = A₀ e^(-kt)   (1)

Where:

A and A₀: concentrations or mass of the compounds, (final and initial)

k: constant decay of the compound

t: given time

Now to get the value of k, we should use the following expression:

k = ln2 / t₁/₂   (2)

You should note that this expression is valid when the reaction is of order 1 or first order. In this kind of exercises, we can assume it's a first order because we are not using the isotope for a reaction.

Now, let's calculate k:

k = ln2 / 956.3

k = 7.25x10⁻⁴ d⁻¹

With this value, we just replace it in (1) to get the final mass of the isotope. The given time is 1 year or 365 days so:

A = 250 e^(-7.25x10⁻⁴ * 365)

A = 250 e^(-0.7675)

A = 191.87 g

However, the question is the percentage left after 1 year so:

% = (191.87 / 250) * 100

<h2>% = 76.75%</h2><h2>And this is the % of isotope after 1 year</h2>
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3 years ago
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Which of the answer choices best describes elements? homogeneous mixtures, always solid, pure substances, very dense.
vampirchik [111]
It can be a compound or a single element. An element is a pure substance that cannot be separated into simpler substances by chemical or physical means. There are about 117 elements, but carbon, hydrogen, nitrogen and oxygen are only a few that make up the largest portion of Earth.

I would say pure substances
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Since the oxygen likes to hog all the electrons, it gives the hydrogen molecules a slight positive charge and the itself a slightly negative charge.
Positive attracts negative, so there is some sort of attraction between water molecules, though a weak one.
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The answer is Gas Giants
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