Let's analyze the problem step-by-step.

Problem Restatement

• 3 g of element C combines with 8 g of element D to form compound CD.
• You want to find how many grams of element D are needed to form compound CD₂.

Step 1: Understand the given compound CD

• In compound CD, the ratio of element C to element D by mass is 3 g : 8 g.
• This means for every 3 g of C, there are 8 g of D.

Step 2: Determine the mass ratio of C to D in CD

• Mass ratio (C:D) = 3 : 8

Step 3: For compound CD₂

• The formula indicates 1 atom of C combines with 2 atoms of D.
• Since the ratio of atoms changes, the mass ratio will also change accordingly.

Step 4: Calculate the mass of D in CD₂

• In CD, the mass ratio corresponds to 1 atom of C and 1 atom of D.
• So, mass of C corresponds to 3 g, mass of D corresponds to 8 g.
• For CD₂, there are 2 atoms of D for every 1 atom of C.
• Therefore, mass of D needed = 2 × 8 g = 16 g.

Final Answer:

To form compound CD₂, 16 grams of element D are needed to combine with 3 grams of element C. If you want, I can also help you with the molar masses and more detailed stoichiometric calculations!