Stoichiometry, Molarity, and Solution Concentrations

Flashcards testing knowledge of formula weights, percent composition, Avogadro's number, molarity calculations, electrolyte ion concentrations, dilutions, and alternative concentration expressions.

What is the formula weight (FW) of a chemical compound?

The sum of the atomic weights for all the atoms in its chemical formula.

What is the formula weight of sucrose ($C_{12}H_{22}O_{11}$)?

$$342.0\,\text{amu}$$

What is the formula weight of calcium nitrate ($Ca(NO_3)_2$)?

$$164.1\,\text{amu}$$

What is the general formula for calculating the mass percentage of an element in a compound?

$$\% \text{ Element} = \frac{(\text{number of atoms})(\text{atomic weight})}{\text{FW of the compound}} \times 100$$

What is the mass percentage of carbon in ethane ($CH_3CH_3$)?

$$79.887\%$$

What chemical standard is Avogadro's number ($6.0221421 \times 10^{23}$) based upon?

It is defined as the exact number of atoms in $12\,\text{g}$ of $^{12}C$.

How many hydrogen atoms are contained in $0.350\,\text{mol}$ of glucose ($C_6H_{12}O_6$)?

$$2.53 \times 10^{24}\,\text{H atoms}$$

How is molar mass defined, and what are its standard units?

Molar mass is the mass of $1\,\text{mol}$ of a substance, expressed in units of $\text{g/mol}$.

How does the molar mass of a substance in grams per mole relate to its formula weight in atomic mass units?

The molar mass in $\text{g/mol}$ is numerically equal to its formula weight in $\text{amu}$.

How many moles of glucose ($C_6H_{12}O_6$) are in $5.380\,\text{g}$ of $C_6H_{12}O_6$ given a molar mass of $180.0\,\text{g/mol}$?

$$0.02989\,\text{mol } C_6H_{12}O_6$$

What is the mass in grams of $0.433\,\text{mol}$ of calcium nitrate ($Ca(NO_3)_2$), given its formula weight of $164.1\,\text{amu}$?

$$71.1\,\text{g } Ca(NO_3)_2$$

How many oxygen atoms are present in a $5.23\,\text{g}$ sample of glucose ($C_6H_{12}O_6$)?

$$1.05 \times 10^{23}\,\text{atoms O}$$

What is the empirical formula of para-aminobenzoic acid (PABA) if it is composed of $61.31\%$ C, $5.14\%$ H, $10.21\%$ N, and $23.33\%$ O?

$$C_7H_7NO_2$$

What organic compound corresponds to the structural formula shown below?

para-aminobenzoic acid (PABA)

In a chemical solution, what is the distinction between the solvent and a solute?

The solvent is the component present in the greatest amount, whereas solutes are all other components dispersed uniformly throughout the solvent.

What is the formula for molarity ($M$), and what is a common student misconception regarding its definition?

$\text{Molarity } (M) = \frac{\text{moles of solute}}{\text{volume of solution in liters}}$. A common misconception is calculating moles per liter of solvent rather than per liter of solution.

What are the three laboratory steps depicted below for preparing $0.250\,\text{L}$ of a $1.00\,M$ $CuSO_4$ solution?

1. Weigh out $39.9\,\text{g}$ ($0.250\,\text{mol}$) $CuSO_4$. 2. Put $CuSO_4$ into a $250\text{-mL}$ volumetric flask, add water, and swirl to dissolve solute. 3. Add water until solution just reaches calibration mark on neck of flask.

What is the molarity of a solution made by dissolving $23.4\,\text{g}$ of sodium sulfate ($Na_2SO_4$) in enough water to form $125\,\text{mL}$ of solution?

$1.32\,M$ (or $1.32\,\text{mol Na}_2\text{SO}_4/\text{L soln}$)

What are the respective molar concentrations of $Na^+$ ions and $SO_4^{2-}$ ions in a $1.0\,M$ solution of $Na_2SO_4$?

$2.0\,M$ in $Na^+$ ions and $1.0\,M$ in $SO_4^{2-}$ ions.

According to the diagram shown, what conversion factors directly relate Grams to Moles, and Moles to Formula units?

Molar mass connects Grams and Moles, while Avogadro's number connects Moles and Formula units.

How many moles of $Na_2SO_4$ are needed to prepare $0.350\,\text{L}$ of a $0.500\,M$ $Na_2SO_4$ solution?

$$0.175\,\text{mol } Na_2SO_4$$

What equation expresses the relationship between concentrations and volumes during a dilution, and why is it valid?

$M_c \times V_c = M_d \times V_d$, valid because adding solvent leaves the total number of moles of solute unchanged.

How many milliliters of $3.0\,M$ $H_2SO_4$ are required to make $450\,\text{mL}$ of $0.10\,M$ $H_2SO_4$ solution?

$$15\,\text{mL}$$

What is the mathematical equation for calculating the mass percentage of a solute A in solution?

$$\text{Mass \% of A} = \frac{\text{mass of A in solution}}{\text{total mass of solution}} \times 100$$

What are the mathematical formulas for concentration in parts per million (ppm) and parts per billion (ppb)?

$\text{ppm} = \frac{\text{mass of A in solution}}{\text{total mass of solution}} \times 10^6$ and $\text{ppb} = \frac{\text{mass of A in solution}}{\text{total mass of solution}} \times 10^9$