Arrange the given substances in decreasing order of mass in grams.
P - One atom of silver
Q - Two gram atoms of nitrogen
R- One mole of calcium
S - Two grams of sodium
(At. masses : Ag = 108 u, N=14 u, Ca=40 u, Na = 23 u)

Options

A. `R gt Q gt S gt P`
B. `Q gt S gt P gt R`
C. `P gt Q gt R gt S`
D. `S gt R gt Q gt P`

Answer

To arrange the given substances in decreasing order of mass in grams, we will first calculate the mass of each substance based on the provided atomic masses.

### Step 1: Calculate the mass of one atom of silver (P)
- The atomic mass of silver (Ag) is 108 u.
- The mass of one atom of silver in grams can be calculated using Avogadro's number (6.02 x 10^23 atoms/mole):

\[ \text{Mass of one atom of Ag} = \frac{108 \text{ g/mol}}{6.02 \times 10^{23} \text{ atoms/mol}} \approx 1.79 \times 10^{-22} \text{ g} \]

### Step 2: Calculate the mass of two gram atoms of nitrogen (Q)
- One gram atom of nitrogen (N) is equivalent to one mole, which has a mass of 14 g.
- Therefore, two gram atoms (or two moles) of nitrogen will have a mass of:

\[ \text{Mass of 2 gram atoms of N} = 2 \times 14 \text{ g} = 28 \text{ g} \]

### Step 3: Calculate the mass of one mole of calcium (R)
- The atomic mass of calcium (Ca) is 40 u, which means one mole of calcium has a mass of:

\[ \text{Mass of 1 mole of Ca} = 40 \text{ g} \]

### Step 4: Identify the mass of two grams of sodium (S)
- The problem states that there are two grams of sodium, so:

\[ \text{Mass of 2 g of Na} = 2 \text{ g} \]

### Step 5: List the masses calculated
- P: 1.79 x 10^-22 g (mass of one atom of silver)
- Q: 28 g (mass of two gram atoms of nitrogen)
- R: 40 g (mass of one mole of calcium)
- S: 2 g (mass of two grams of sodium)

### Step 6: Arrange the substances in decreasing order of mass
From the calculations:
- R (40 g) > Q (28 g) > S (2 g) > P (1.79 x 10^-22 g)

### Final Arrangement:
1. R - One mole of calcium (40 g)
2. Q - Two gram atoms of nitrogen (28 g)
3. S - Two grams of sodium (2 g)
4. P - One atom of silver (1.79 x 10^-22 g)