Question

1. the percent by mass of water in the hydrate $ce{na_{2}so_{4}cdot 10h_{2}o}$ is closest to

a. 18% b. 44% c. 56% d. 76%

1. base your answer(s) to the following question(s) on the information below.

arsenic is often obtained by heating the ore arsenopyrite, feass. the decomposition of feass is represented by the balanced equation below.
$ce{feass(s) xrightarrow{heat} fes(s) + as(g)}$
in the solid phase, arsenic occurs in two forms. one form, yellow arsenic, has a density of $1.97 \text{g/cm}^3$ at stp. the other form, gray arsenic, has a density of $5.78 \text{g/cm}^3$ at stp. when arsenic is heated rapidly in air, arsenic(iii) oxide is formed.
although arsenic is toxic, it is needed by the human body in very small amounts. the body of a healthy human adult contains approximately 5 milligrams of arsenic.
calculate the percent composition by mass of arsenic in arsenopyrite. your response must include both a correct numerical setup and the calculated result.

1. base your answers to the following questions on the information below.

vitamin c, also known as ascorbic acid, is water soluble and cannot be produced by the human body. each day, a persons diet should include a source of vitamin c, such as orange juice. ascorbic acid has a molecular formula of $ce{c_{6}h_{8}o_{6}}$ and a gram-formula mass of 176 grams per mole.
in the space, show a numerical setup for calculating the percent composition by mass of oxygen in ascorbic acid.

Explanation:

Step1: Calculate molar mass of hydrate

First, find molar masses:
$M(\text{Na}_2\text{SO}_4) = 2\times23 + 32 + 4\times16 = 142\ \text{g/mol}$
$M(10\text{H}_2\text{O}) = 10\times(2\times1 + 16) = 180\ \text{g/mol}$
$M(\text{Na}_2\text{SO}_4\cdot10\text{H}_2\text{O}) = 142 + 180 = 322\ \text{g/mol}$

Step2: Calculate mass percent of water

$\text{Mass percent} = \frac{M(10\text{H}_2\text{O})}{M(\text{Na}_2\text{SO}_4\cdot10\text{H}_2\text{O})} \times 100\%$
$\text{Mass percent} = \frac{180}{322} \times 100\% \approx 55.9\%$

Step1: Calculate molar mass of FeAsS

Molar masses: $M(\text{Fe})=56\ \text{g/mol}$, $M(\text{As})=75\ \text{g/mol}$, $M(\text{S})=32\ \text{g/mol}$
$M(\text{FeAsS}) = 56 + 75 + 32 = 163\ \text{g/mol}$

Step2: Calculate mass percent of As

$\text{Mass percent} = \frac{M(\text{As})}{M(\text{FeAsS})} \times 100\%$
$\text{Mass percent} = \frac{75}{163} \times 100\% \approx 46.0\%$

Step1: Find total mass of oxygen in 1 mole

In $\text{C}_6\text{H}_8\text{O}_6$, there are 6 O atoms: $M(\text{O})=16\ \text{g/mol}$
Total mass of O: $6\times16 = 96\ \text{g}$

Step2: Write mass percent setup

$\text{Mass percent of O} = \frac{\text{Total mass of O}}{\text{Gram-formula mass of ascorbic acid}} \times 100\%$

Answer:

C. 56%

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