Expressed in the correct number of significant figures the sum of 0.53 cm + 75.321 cm + 28.1 cm =

Chemistry

1 answer:

Nadya [2.5K]3 years ago

5 0

Answer:

$\boxed{\text{104.0 cm}}$

Explanation:

When adding or subtracting values, you must round your answer to the same "place" as the measurement with its last significant figure furthest to the left.

That is, you round off to the same number of decimal places as the measurement with the fewest decimal places.

$\begin{array}{r|l}0.5& \text{3 cm}\\+ 75.3& \text{21 cm}\\+ 28.1 & \quad\text{cm} \\= 103.9 &\text{51 cm}\\\end{array}$

The measurement of 28.1 cm has one digit after the decimal point, so you round the sum to have only one digit to the right of the decimal.

However, the number to be dropped (51) is greater than 5 with only zeros or nothing following the 5.

You must round up the answer. You increase the last significant digit by one.

$\text{The sum is $\boxed{\textbf{104.0 cm}}$}$

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Which of the following reactions have a positive ΔSrxn? Check all that apply.

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Answer:

The reactions that have a positive ΔS rxn are the first and the fourth choices:

2A(g) + B(s) → 3C(g)

2A(g) + 2B(g) → 5C(g)

Explanation:

ΔS rxn is the change of entropy of the chemical reaction.

ΔS rxn = S after reaction - S before reaction.

Therefore, a positive ΔS rxn means that the entropy after the reaction is greater than the entropy before the reaction.

You may use some assumptions to predict whether a reaction will lead an increase or decrease of the entropy.

First, assume that all the non-shown conditions, such as temperature and pressure, are constant.

Under that assumption, and from the meaning of entropy as a measure of the disorder or randomness of a system you can predict the sign of the change of entropy.

2A(g) + B(s) → 3C(g)

1) The solid compounds, B(s) in this case, are very ordered and so they have low entropy.

2) Gas molecules are highly disordered (scattered), and the greater the number of molecules of the gas the larger the entropy, S).

Hence, since the product side shows 3 gas molecules and the reactant side shows 2 gas molecules and 1 solid molecule, you predict that the products have a larger entropy than the reactants, meaning an increase in entropy: ΔS rxn is positive.

2A(g) + B(g) → C(g)

Using the same reasoning, 3 gas molecules in the reactant side have more entropy than 1 molecule in the product side, and so the reaction leads to a decrease in the entropy: ΔS rxn is negative

A(g) + B(g) → C(g)

Again, 2 gas molecules in the reactant side have more entropy than 1 molecule in the product side, and so the reaction leads to a decrease in the entropy: ΔS rxn is negative

2A(g) + 2B(g) → 5C(g)

With the same reasoing, 5 molecules in the product side, lets you predict that will have more entropy than 4 molecules in the reactant side, and, the entropy will increase: ΔS rxn is positive.

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Answer: 2!

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How many sulfur atoms would be present in 41.8 moles of sulfur?

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