Question

what is the mass of 0.312 mol of hydrazine, n₂h₄ (molar mass = 32.06 g/mol).
10.0 grams
0.00973 grams
0.0163 grams
23.1 grams
question 20
how many carbon atoms are present in 20.0 mg of saccharin (c₇h₅no₃s with a molar mass of 183.18 g/mol)?
4.59x10²⁰ c atoms
9.17x10²⁰ c atoms
6.83x10²³ c atoms
6.02x10²³ c atoms

Explanation:

Question 1

Step1: Recall mass - mole formula

The formula is $m = n\times M$, where $m$ is mass, $n$ is the number of moles, and $M$ is the molar mass.

Step2: Substitute values

Given $n = 0.312$ mol and $M=32.06$ g/mol. Then $m=0.312\times32.06$ g.
$m = 0.312\times32.06=9.99272\approx10.0$ g

Step1: Convert mass to grams

$20.0$ mg is converted to grams. Since $1$ g = $1000$ mg, $m = 20.0\times10^{- 3}$ g.

Step2: Calculate number of moles of saccharin

Use the formula $n=\frac{m}{M}$. Here, $m = 20.0\times10^{-3}$ g and $M = 183.18$ g/mol, so $n=\frac{20.0\times10^{-3}}{183.18}$ mol.

Step3: Determine number of moles of carbon

In one molecule of $C_7H_5NO_3S$, there are 7 carbon atoms. So the number of moles of carbon $n_{C}=7\times n$, where $n$ is the number of moles of saccharin.

Step4: Calculate number of carbon atoms

Use Avogadro's number $N = n_{C}\times N_A$, where $N_A=6.022\times 10^{23}$ atoms/mol.
$n=\frac{20.0\times10^{-3}}{183.18}\approx0.00010917$ mol
$n_{C}=7\times0.00010917 = 0.00076419$ mol
$N=0.00076419\times6.022\times 10^{23}\approx4.59\times 10^{20}$ atoms

Answer:

10.0 grams

Question 2