Units of hardness:\r One degree of hardness (Clark's degree) corresponds approximately to what concentration of CaCO3 equivalent in mg/L?

Introduction / Context:
Hardness is often expressed in different traditional units. Converting among these is essential for comparing standards, plant data, and research reports. Clark's degree (°Clark) is an older British unit still encountered in practice.

Given Data / Assumptions:

• We are asked for the CaCO3 equivalent concentration corresponding to 1 °Clark.
• Standard conversion: 1 °Clark ≈ 1 grain per Imperial gallon of CaCO3 equivalent.

Concept / Approach:
Using known conversions, 1 grain per Imperial gallon equals approximately 14.25 mg/L as CaCO3. This value is widely cited and used to translate historical data into SI-based concentrations.

Step-by-Step Solution:
Recall: 1 °Clark = 1 grain per Imp. gallon as CaCO3.Convert to mg/L: ≈ 14.25 mg/L as CaCO3.Therefore, select 14.25 mg/L as the correct equivalence.

Verification / Alternative check:
Contrast with other units: 1 °German (°dH) ≈ 17.8 mg/L as CaCO3, and 1 °French (°f) = 10 mg/L as CaCO3. The stated 14.25 mg/L aligns with °Clark, distinguishing it from these other scales.

Why Other Options Are Wrong:

• 10.25 and 12.25 mg/L: Too low; closer to the French degree is 10 mg/L.
• 16.25 and 20.00 mg/L: Too high, and 17.8 mg/L corresponds to German degrees, not Clark.

Common Pitfalls:

• Mixing up German, French, and Clark scales.
• Reporting hardness without specifying the unit basis (as CaCO3).

Final Answer:
14.25 mg/L as CaCO3