Solve a problem

Moles and massChemistryHigh schoolabout 1:21

How many moles are in 15.8 g of KMnO4?: animated lesson

A narrated, step-by-step moles and mass video lesson with captions. Final answer: M(KMnO4) = 158.032 g/mol; 15.8 g = 0.0999798 mol.

SpeedLanguage

Chapters

Transcript

Video with a synthetic English voice and captions. Pick another language for subtitles in that language, or open the interactive version.

Video narrated by a synthetic voice (Piper TTS, en_US-ljspeech-high voice (LJ Speech dataset, public domain)); the steps come from our maths engine and the answer was checked independently. Download MP4 (0.6 MB) · Captions (WebVTT)

Steps on screen

1. The problem
Worked example
How many moles are in 15.8 g of KMnO4?
2. Steps, part 1
1. $\text{K}: 1,\quad \text{Mn}: 1,\quad \text{O}: 4$
  Count the atoms of each element
2. $\text{K} = 39.098,\quad \text{Mn} = 54.938,\quad \text{O} = 15.999$
  Look up each atomic weight (g/mol)
3. $1 \times 39.098 = 39.098 \\ 1 \times 54.938 = 54.938 \\ 4 \times 15.999 = 63.996$
  Multiply each count by its atomic weight
4. $M = 39.098 + 54.938 + 63.996 = 158.032\ \text{g/mol}$
  Add the contributions
3. Steps, part 2
1. $\%\text{K} = \frac{39.098}{158.032} \times 100 \approx 24.74\% \\ \%\text{Mn} = \frac{54.938}{158.032} \times 100 \approx 34.76\% \\ \%\text{O} = \frac{63.996}{158.032} \times 100 \approx 40.50\%$
  Mass percent of each element
2. $n = \frac{m}{M} = \frac{15.8\ \text{g}}{158.032\ \text{g/mol}} \approx 0.0999798\ \text{mol}$
  Convert the sample mass to moles
3. $N = n N_A \approx 0.0999798 \times 6.02214076\times10^{23} \approx 6.0209\times10^{22}$
  Number of formula units
4. The answer
$M = 158.032\ \text{g/mol}$
1×39.098 + 1×54.938 + 4×15.999 = 158.032 g/mol
5. Check
- Percent composition adds to 100.00%.
- Elements: potassium ×1, manganese ×1, oxygen ×4.

Transcript

The problem. Here is the problem: How many moles are in 15.8 g of KMnO4? Add the atomic weights of all 6 atoms in one formula unit of KMnO4.

Steps, part 1. Count the atoms of each element. Subscripts multiply everything inside the brackets before them, and a hydrate coefficient multiplies the whole water (or other) unit. Look up each atomic weight (g/mol). IUPAC standard atomic weights (abridged, conventional values). Multiply each count by its atomic weight. Add the contributions.

Steps, part 2. Mass percent of each element. Convert the sample mass to moles. Number of formula units.

The answer. So the answer is M(KMnO4) equals 158.032 g/mol; 15.8 g equals 0.0999798 mol. The molar mass is the mass of one mole of formula units. It equals the sum of the atomic weights of every atom in the formula, and numerically matches the formula mass in daltons (u).

Check. Percent composition adds to 100.00 percent. Elements: potassium times 1, manganese times 1, oxygen times 4.

About moles and mass

Convert between grams, moles and particles with molar mass and Avogadro's number.

How many moles are in 15.8 g of KMnO4?: Step-by-Step Lesson